482 Comparative Morphology of Chordates 



of the eleventh and twelfth nerves indicate that they correspond, at 

 least in part, to certain anterior spino-occipital nerves of fishes and 

 amphibians, but the accessory (XI) seems to have annexed some 

 posterior roots of the vagus (X) . 



The reproductive methods which prevail among anamniotes are 

 quite unsuited to terrestrial animals. Although amphibians are to 

 some extent adapted for living on land, the gilled larval stage in their 

 development forbids their living far from fresh water. Some of them 

 have found means (see p. 239) of passing the larval stage without 

 recourse to external water, and a few of them omit the "tadpole" 

 stage. The great majority of them, if not wholly aquatic, must seek 

 water at the breeding season. 



Vertebrate eggs range in size from the microscopic to the size of 

 ostrich eggs. The eggs of anamniotes, with some exceptions, are well 

 toward the lower end of the range. Small eggs contain a correspond- 

 ingly small amount of food material for the building up of the embryo. 

 Therefore the small anamniote egg, usually deposited and left to its 

 fate in the open water, must develop rapidly into an animal which, 

 while still very minute, has all of its organs (except the reproductive) 

 developed to a functional state so that, as soon as the food within the 

 egg is exhausted, the animal will be able to feed itself and begin its 

 independent life. Then follows a prolonged period of growth before 

 the animal, if ever, attains sexual maturity and adult size. If a python 

 or dinosaur were to hatch out of its egg and begin free life on land at a 

 length of 2 or 3 mm., the size of many a newly hatched fish, it could 

 hardly hope to grow up. 



Eggs of reptiles, compared to those of most anamniotes, are of 

 enormous size. They are fertilized (by copulation) before they are 

 laid. The copulatory organ (penis) in turtles, tortoises, and croco- 

 dilians is a median longitudinal thickening of the ventral wall of the 

 cloaca. It is vascular, erectile, and protrusible through the cloacal 

 aperture. A groove along its dorsal surface conducts the sperm. 

 Lizards and snakes have a pair of protrusible cloaca! structures, the 

 hemipenes. 



The eggs are invested by protective shells of a tough leathery tex- 

 ture, or hard like shells of birds' eggs. They are deposited on land 

 (except in viviparous species). In contrast to the usual anamniote 

 development in which differentiation of organs is early and rapid but 

 accompanied by little growth, the reptile has a prolonged prehatching 

 period during which differentiation proceeds, so to speak, at leisure, 

 and relatively great size is attained before hatching. A newly hatched 

 alligator is about 8 inches long. The early embryo surrounds itself by 

 a system of membranes produced from its own tissues (see p. 291). The 



