64 STRUCTURE OF THE VERTEBRATES 



snakes (Crotalus) ; the ''copper head" and the "cotton mouth 

 moccasin", the last two belonging to the same genus (Agkis- 

 trodon) . 



The smallest snakes are only a few inches in length, the largest 

 more than thirty feet. The latter are the python of the Old 

 World, and the anaconda of America. Both are tropical in range, 

 and not poisonous. They throw coils around their prey and con- 

 strict until the animal is dead. 



Embryology of the Reptiles. Fertilization in the reptiles is 

 always internal, and takes place before the membranes and shell 

 are laid around the yolk. As the egg approaches maturity in the 

 ovary yolk granules develop, and when the egg is mature it is 

 large and heavily loaded with yolk. This inert material is cen- 

 tered at the ventral pole, with the active protoplasm only a small 

 drop located at the dorsal side. It is apparent that the pro- 

 toplasm cannot exert sufficient force to cleave the entire mass, 

 and the result is that the cleavages are limited to the drop of 

 protoplasm. This type of cleavage is called merohlastic, as op- 

 posed to the holoblastic cleavage of eggs which divide entirely, 

 as illustrated by Amphioxus and the frog. 



The first and second cleavages cut the protoplasm, or germinal 

 disc, at right angles to each other. But the protoplasm is so thin 

 that transverse cleavage is impossible, and the third cleavage is 

 in the same direction as the first two. The result is that the eight 

 cells are in one horizontal plane. Future divisions continue to 

 cut the disc, until a blastula is formed as a flat plate of cells 

 spread out over the yolk. The student can visualize this and 

 later processes by breaking a fertilized hen's egg and looking 

 for a white spot about three millimeters in diameter on top of the 

 yolk. If the slightest bit of red appears it shows that the blood 

 vessels have developed and the embryo is fairly well advanced. 

 The average kitchen is an excellent embryological laboratory if 

 a hand lens is available. 



Gastrulation is due to the ventral growth of cells at one point 

 on the blastula. Comparing Amphioxus and the frog, it will be 

 seen that gastrulation in the latter proceeds almost entirely by 

 the overgrowth of dorsal pole cells, invagination of the ventral 

 cells playing a minor part. The reptile and bird have carried the 



