526 THE DEVELOPMENT OF THE INDIVIDUAL 



organs can develop. The mesoderm is produced from the endoderm by- 

 means of paired pouches. These form on the dorso-lateral margins of 

 the gut (archenteron) and gradually pinch off from the endoderm until 

 they lie free between the ectoderm and the endoderm. The pouches 

 extend both dorsally and ventrally until they meet above and below the 

 digestive tube and in this way form the dorsal and ventral mesenteries. 

 The pouches later connect with each other and form a single cavity, the 

 coelom. At about the same time a third evagination of the endoderm 

 occurs along the dorsal side of the gut. This pinches off and forms a 

 long slender rod of cells which develops into the notochord. Since it is 

 formed at the same time and in the same way as the mesoderm, most 

 embryologists agree that the notochord should also be considered as a 

 mesodermal structure. 



Variations in Cleavage of the Chordate Zygote 



The size of the egg, the amount of yolk, and the distribution of the 

 yolk all influence the exact method of cleavage of the zygote. The 

 method which we have described applies where there is a relatively small 

 amount of yolk, and that which is present is evenly distributed in the 

 zygote. In some chordates (amphibians, cyclostomes, and some primi- 

 tive fishes such as the gars) the amount of yolk is somewhat large, and 

 it is concentrated in the half of the zygote around the vegetal pole. 

 This half of the zygote will be light in color and is known as the vegetal 

 hemisphere, whereas the darker half will be the animal hemisphere. 

 The first two divisions are equal and split the zygote into four cells, 

 each of which includes one fourth of the animal and one fourth of the 

 vegetal hemispheres. The third division, however, cuts across the 

 zygote up nearer the animal pole, and the four cells of this region are 

 smaller than those at the vegetal pole. The yolk at the vegetal pole 

 seems to interfere with the division of the cells, and the cells at this 

 region divide more slowly than those at the animal pole. Consequently, 

 as division continues there are more and smaller cells around the ani- 

 mal pole. As the blastula is formed we find that the blastocoel is nearer 

 the animal pole than the vegetal pole. When time comes for formation 

 of the gastrula, we find that the cells at the vegetal pole are so large that 

 there can be little invagination as was found in Amphioxus. There is 

 a slight invagination, however, and then the cells from the animal hemi- 

 sphere begin growing down over the sides of the vegetal hemisphere. 

 In time they completely cover this vegetal hemisphere with the excep- 

 tion of a small round area. This area is the equivalent of the blasto- 



