408 AN INTRODUCTION TO ZOOLOGY 



The tops of the neural folds as they come together are somewhat 

 flattened, so that when they join, a portion of the nervous matter is 

 left under the superficial ectoderm along the sides of the line of 

 junction. This does not take part in the formation of the tube 

 itself, but is left as a pair of longitudinal bands on the dorso-lateral 

 sides of the neural tube. These are the neural crests, and will give 

 rise to the ganglia of the cranial and spinal nerves. 



We have thus followed the development of the chick up to 

 a stage corresponding fairly closely with those at which we left 

 Amphioxus and Rana, and in spite of the considerable differences 

 in the method of attaining it, owing to the presence of a large amount 

 of yolk, the condition of the embryo in all three species shows a 

 remarkable fundamental resemblance. 



Lepus. 



The study of the development of the mammalian ovum is 

 one that presents a number of difficulties. In the first place the 

 ovum itself is of small size, being about '2 mm. in diameter in man and 

 17 mm. in the dog and the rabbit ; this makes it hard to find and 

 manipulate when obtained. Secondly, save in the case of the rabbit, 

 there is no definite relationship between ovulation and any external 

 happening, so that it is largely a matter of chance if early stages 

 are obtained. In the rabbit ovulation occurs q or 10 hours after 

 copulation, so that in this species one difficulty is partly removed. 

 Then, too, the mammalian egg has undoubtedly been derived from a 

 large heavily yolked telolecithal egg, somewhat like that of the fowl. 

 In spite of its small size and the fact that it is almost devoid of yolk, 

 it follows in its development the same general course as a telolecithal 

 egg and does not return to the primitive homolecithal type. Lastly, 

 the segmenting ovum, deprived of yolk as source of food supply, 

 takes on, at quite an early stage, a definite and very intimate connec- 

 tion with the wall of the uterus, whence it obtains its nutriment. 

 This profoundly modifies the early stages of development, and so 

 we find a number of processes occurring, particularly in the early 

 stages of segmentation, that either have no counterpart in other 

 forms or else our knowledge of them is insufficient to allow of definite 

 homologies being established. 



The extrusion of the first polar body occurs while the 

 ovum is still in the graafian follicle. When the latter has become 

 fully formed, it slowly enlarges until it is transformed into a hollow 

 vesicle filled with follicular fluid and having its walls lined by a layer 

 several cells thick, the membrana granulosa. To one side of this 

 the ovum is attached by means of a group of cells surrounding it. 



