EMBRYOLOGY OF THE CHICK 



With what has just been said in mind, the developmental processes 

 of an embryo become more understandable." The unicellular germ disc 

 is composed of a very definite area at the animal pole. The disc itself 

 is about three millimeters in diameter, and less than five-tenths milli- 

 meters in thickness. Directly beneath this disc, there is a merging of 

 the protoplasm with the white yolk. This well-marked region is called 

 the nucleus of Pander (Fig. 251, P), and this connects the central white 

 yolk by a narrow stalk called the latebra. It is necessary to study all 

 the figures carefully to understand these and successive terms, and to 

 grasp the relationship of each to the other. 



There are two regions in the disc itself : the larger central portion 



in 



Fig. 257. 



Cleavage. Upper Row, Amphioxus. (After Hatschek. ) 1, Unfertilized egg; 2, 

 stage of two blastomeres ; 3, stage of four blastomeres ; 4, stage of eight blast- 

 omeres ; 5, stage of seventy-two blastomeres ; 6, section of blastula ; p.b., polar body. 

 Middle Row, Frog. B, segmentation cavity, v, nucleus. Lower Row, Hen's egg. 

 (After Patterson.) Surface views of the blastoderm and the inner part of the 

 marginal periblast only. The anterior margin of the blastodisc is toward the top 

 of the page. A. Two-cell stage, About three hours after fertilization. B. Four 

 cells. About three and one-fourth hours after fertilization. C. Eight cells. About 

 four hours after fertilization. D. Thirty-four cells. About four and three-fourths 

 hours after fertilization. E. One hundred and fifty-four cells upon the surface ; 

 the blastoderm averages about three cells in thickness at this stage. About seven 

 hours after fertilization, ac. Accessory cleavage furrows ; m, radial furrow ; p, 

 inner part of marginal periblast ; sac, small cell formed by the accessory cleavage 

 furrows. 



