STRUCTURE OF THE VERTEBRATES 53 



tioned, is used to illustrate an intermediate stage between the 

 amphibia and the higher vertebrates. 



The student is undoubtedly familiar with the processes of 

 maturation of the egg and sperm. The mature eggs of frogs and 

 toads are fertilized by the male at the time of deposition, before 

 the thin coating of gelatinous material swells by the addition of 

 water. The toad lays its eggs in a string, the frog and salamander 

 eggs are laid in masses. The frog egg is spherical, about one 

 millimeter in diameter, and has more inert yolk material than is 

 found in Amphioxus. These yolk granules are also more concen- 

 trated at the ventral pole. The plans of the first two cleavages 

 are dorso-ventral, and that of the third is transverse. This is 

 similar to the first three cleavages of Amphioxus, with the slight 

 exception that the third is more asymmetrical, the dorsal four 

 cells of the frog being decidedly smaller than the ventral. The 

 general principle that a cell loaded with inert matter cleaves 

 more slowly than one with relatively more protoplasm has been 

 stated; therefore in the frog cleavages after the fourth proceed 

 more rapidly in the dorsal region of the embryo. The result is a 

 blastula with the segmentation cavity entirely in the upper half 

 of the egg. 



Gastrulation in Amphioxus was described as due to both in- 

 vagination and the overgrowth of the rapidly dividing dorsal 

 cells. In the frog invagination is largely suppressed, and the 

 latter process greatly increased. This overgrowth is more rapid at 

 the anterior end. The student can visualize the process if he will 

 think of the equator of the egg remaining static while the 

 rapidly dividing upper cells flow ventrally as a double layer, 

 more rapidly on one end than the other. As this two-layered 

 structure approaches the posterior pole the opening tends to 

 constrict; and when gastrulation is complete, only a narrow 

 aperture is left as the blastopore. From this it is clear that most 

 of the endodermal lining of the archenteron has been derived 

 from the dorsal pole cells. The remainder of the endoderm de- 

 veloped from the invaginating ventral pole cells. During gas- 

 trulation the segmentation cavity was practically crowded out 

 of existence. 



At the completion of gastrulation the process of elongation 

 begins, and then the dorsal neural groove is formed. The hollow 



