After fertilization, the frog zygote proceeds through radial, holoblastic cleavage. Because of the large mass of yolk in the vegetal pole, the resulting blastomeres are of unequal sizes yeilding micromeres (1) at the animal pole and macromeres (2) at the vegetal pole with evident cleavage furrows (3) between them. Each one of these resulting cells contain a nucleus (4), which are often missed in the thin, cross sectional cuts. At the center of the blastomeres one can see a cavity beginning to form called the blastocoel (5). It's obvious that fertilization has occured because cleavage is occuring (duh!), but also because the fertilization envolope (6) is visibly raised from the surface of the dividing cells. Along the periphery (especially at the animal pole) you can see the layer of cortical pigments (7). This protects the eggs and early stages of development from direct sunlight. The yolk itself is composed of golgi-derived yolk platelets (8).
Sometimes referred to as "late cleavage," the blastula is a hollow ball of cells. The cells at this stage continue to undergo rapid cell division (mitosis-1) creating a hollow space within the blastula called the blastocoel (2). This transient structure which will soon disappear during gastulation performs the short but important task of preventing cell-cell communication between the ectoderm at the animal pole and the endoderm below in the vegetal pole. The point of sperm entrance will determine the anterior (A) and posterior (P) axis in the embryo. We can estimate the quadrant of sperm entrance based on the cortical layer (3). The counterclockwise rotation seen by the lower left margin of cortical layer (4) indicates that sperm entrance occured in the left quadrant of the animal pole (the cortical layer rotates toward the point of sperm entrance). Invagination will begin opposite of sperm entrance (5).
As expected, gastrulation begins with the invagination of the blastopore (1) opposite of sperm entrance. Throughout gastrulation, the ectoderm will proliferate causing the surface of the blastula to slide towards the blastopore (epiboly-2) and in (involution-3) to form the gut precursor or archenteron(4). At this stage the archenteron is little more that a slit and the blastocoel (5) is still very prominent; however, as gastrulation proceeds, the archenteron will enlarge and the blastocoel will be displaced to the side and eventually disappear...it's job is complete. Because frogs are deuterostomes, like other vertebrates, the blastopore marks the posterior (P) region where the proctodeum/anus will form, with the stomodeum forming later in the anterior (A) region. The dorsal lip (6) is clearly visible, at the beginning of gastrulation, while the ventral lip will form later. BTW, what's the purpose of gastrulation? Keep reading.
The purpose of gastrulation is the formation of the three primary germ layers: endoderm, mesoderm, and ectoderm. During late gastrulation all three of these are clearly visible with ectoderm (1) surrounding the outer portion of the embryo, endoderm (2) surrounding the gut (archenteron) and composing the yolk, and mesoderm (3) inbetween. Notice that the blastocoel (4) has been pushed off to one side by the enlarging archenteron (5) and will not be seen again after this. Both the dorsal lip (6) and blastopore (7) are still visible with the addition of the ventral lip (8). The yolk plug (9) is the endodermal mass between the dorsal and ventral lips.
Up to this point each of the other early development slides have been sagittal sections with anterior and posterior ends visible. Look at the neurula...where is anterior and posterior? You don't see them because this is a cross section. The neurula will not make sense to you if you do not understand this!
After gastrulation things start to happen fast with neurulation, the formation of the neural tube. This cross section shows an embryo in the middle of neurulation with the neural folds (1) rising to form the neural tube. On either side you can see the neural crest cells (2) beginning migration. Also present at this stage are all three germ layers: ectoderm (3), endoderm (4) and mesoderm. The mesoderm can be subdivided into 5 regions, 3 of which are shown here: lateral plate mesoderm (A.K.A. ventral mesoderm - 5), notochord (A.K.A. chordamesoderm - 6), and paraxial mesoderm (A.K.A. dorsal mesoderm - 7). The other two divisions, head mesenchyme and intermediate mesoderm (A.K.A. middle mesoderm) are not clearly visible. Not to be confuse with the now absent blastocoel, the archenteron (8) is very prominent.