34 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



3. Meroblastic cleavage.— In eggs containing large quantities of yolk onlj 

 the small germinal disk undergoes cleavage. This kind of cleavage is called 

 meroblastic. As a result, a minute disk of cells is produced on the surface of the 

 relatively enormous yolk. A slight split appears between the disk and the yolk, 

 and this corresponds to the segmentation cavity of other developing eggs; this 

 stage is consequently the blastula stage. Meroblastic cleavage is illustrated in 

 Figure 4C, also in P and H, page 436; N, page 316. The disk of cells in the 

 further development expands over the surface of the yolk and is then called the 

 blastoderm. 



C. FORMATION OF THE GASTRULA 



i. In eggs of the Amphioxus type. — In such eggs the vegetal hemisphere 

 begins to bend inward and continues this process of invagination until its wall 

 comes in contact with the wall of the animal hemisphere. An embryo with a 

 wall two cell-layers thick is thus produced.- It is called a gastrula. The outer 

 layer is named the ectoderm and the inner layer the entoderm. Because of their 

 importance in the subsequent development, these layers are referred to as the 

 first two germ layers. The hollow tube of entoderm is called the archenteron or 

 primitive intestine; the cavity of the gastrula is the cavity of the archenteron or 

 gastrocoel; and the opening of the archenteron to the exterior is the blastopore. 

 Note that the blastocoel is eliminated in the production of the gastrula. The 

 formation of the gastrula of Amphioxus is illustrated in Figure 5^4, also in W, 

 page 59; K, page 9; P and H, page 57; N, page 44. Study further the models 

 exhibited in the laboratory. 



2. In eggs of the amphibian type. — In these eggs the cells of the vegetal 

 hemisphere, owing to their content of inert yolk, are unable to bend in as described 

 for Amphioxus, but instead the more active cells of the animal hemisphere grow 

 down over the cells of the vegetal hemisphere, gradually inclosing them. The 

 result is the same as the foregoing, a gastrula being formed. A small portion 

 of the inclosed yolk-bearing cells commonly remains for some time protruding 

 through the blastopore, and is called the yolk plug. 



The formation of the amphibian gastrula is illustrated in Figure $B, and in 

 P and H, page 289; N, page 206. Study further the models of amphibian 

 development illustrating this stage, noting especially the sagittal section of the 

 gastrula. Then obtain a slide bearing a sagittal section of the gastrula and study 

 with the low power of the microscope. The gastrula is slightly elongated in the 

 anteroposterior direction. The side with the thinner wall is the dorsal side; 

 that with the thick wall, the ventral side; the end with an opening is the posterior 

 end ; the opposite end is anterior. The wall consists of two layers each composed 

 of more than one sheet of cells. The outer and thinner layer is the ectoderm, 

 uniform in width over the whole embryo. The inner layer is the entoderm, 

 separated from the ectoderm by a slight space, and very thick ventrally, where 



