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. 



!!* 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 



-* n c 



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

 a, Border of the genii-disc ; fr, 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 {-hell. It results simply in a cleavage of 

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

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

 becomes subsequently enclosed in an appendage to the embryo, the 

 so-called yolk sac, and is gradually consumed as nutritive mat.-! ial. 

 Ju>t as in tin- m.se of the pigmented, animal halt' of the Frog's egg, 

 so also in the case of the 11< -n'> e^, turn it in whatever direction 

 one will, the garm-dipo floats oo t"p. Lean.-- it is the lighter part. 



As in tin- Efaogfl >-^ tin- lir>t plane ol (!-. i\ai:- i- \ntical ami '.egins 

 at the animal pole. ><> in the case of tl. M-M'> e;i: Mi.i:. " 

 a small furrow </) make> it- appearance in the middle of the li>c, 

 and advances from al>,,\.. .1" \\n\vanl in a vertical direction. But 



