34 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



3. Meroblastic cleavage. In eggs containing large quantities of yolk only 

 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 46". The disk of cells produced by meroblastic cleavage is called the 

 blastoderm; in the further development it expands over the surface of the yolk 

 which it eventually incloses. 



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 imagination 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, and also in 

 the various textbooks, as P and H, W, K, and N. Study further the models 

 exhibited in the laboratory. 



2. In eggs of the amphibian type. In these eggs gastrulation is somewhat 

 modified by the presence of inert yolk in the vegetal hemisphere. It is accom- 

 plished partly by the invagination of the entoderm particularly at the dorsal lip 

 of the blastopore, and partly by the expansion of the ectoderm ventrally pushing 

 the entoderm into the interior. The result is the same as the foregoing, a gastrula 

 being formed. A small portion of the inclosed yolk-bearing cells commonly re- 

 mains 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, N, and other textbooks. 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 



