62 



EMBRYOLOGY. 



ends of the embryo may be determined in the fertilised egg. That 

 half of the egg, namely, through which the spermatic nucleus 

 migrates to reach the egg-nucleus, becomes the tail-end of the 

 embryo ; the opposite half becomes the head-end. Every egg, 

 however, can be fertilised in any meridian whatever, as was demon- 

 strable experimentally, and thereby the tail-end of the embryo may 

 be located at any chosen position in the egg. Thirdly, the plane 

 in which the two sexual nuclei meet each other (copulation-plane) 

 corresponds with the first plane of segmentation. 



II a - Partial Discoidal Cleavage. 



The Hen's egg serves us as the classical example for the description 

 of discoidal segmentation. In this instance the whole process of 



ARC 



Fig. 33. Surface view of the first stages of cleavage in the Hen's egg, after COSTE. 

 a, Border of the germ-disc ; b, vertical furrow ; c, small central segment ; d, large peripheral 

 segment. 



cleavage takes place while the egg is still in the oviduct, during the 

 period in which the yolk is being surrounded by the albuminous 

 envelope and the calcareous shell. It results simply in a cleavage of 

 the germ-disc of formative yolk, whereas the greater part of the 

 egg, which contains the nutritive yolk, remains unsegmented, and 

 becomes subsequently enclosed in an appendage to the embryo, the 

 so-called yolk-sac, and is gradually consumed as nutritive material. 

 Just as in the case of the pigmented, animal half of the Frog's egg, 

 so also in the case of the Hen's egg, turn it in whatever direction 

 one will, the germ-disc floats on top, because it is the lighter part. 

 As in the Frog's egg the first plane of cleavage is vertical and begins 

 at the animal pole, so in the case of the Hen's egg (fig. 33 A) 

 a small furrow (6) makes its appearance in the middle of the disc, 

 and advances from above downward in a vertical direction. But 



