866 



STRUCTURES FORMED FROM THE EPIBLAST. 



hollow vesicle (VI), the primary optic vesicle. The remainder of the epiblast 

 forms the epidermal covering of the body. At an early period we can distinguish 

 the stratum comeum and the Malpighian layer of the skin ( 283) ; from the 

 former are developed the hairs, nails, feathers, &c. 



Partial Cleavage Only a partial cleavage takes place in the eggs of birds and in mero- 

 blastic 6va i e only the white yelk in the neighbourhood of the cicatricula divides into nume- 

 rous segmentation spheres (Caste, 1848). The cells arrange themselves in two layers lying one 



Fig. 665. 



The three layers of the blastoderm of a mammalian ovum Z, zona pellucida ; E, epiblast \m, 

 niesoblast ; c, hypoblast. II, Section of an embryo, with six proto vertebrae at the 1st day 

 M, medullary groove ; h, somatopleure ; U, protovertebra ; c, chorda dorsalis ; S, the 

 lateral plates divided into two ; e, hypoblast. Ill, Section of an embryo chick at the 2nd 

 day in the region behind the heart M, medullary groove ; h, outer part of somatopleure ; 

 u, protovertebra ; c, chorda ; w, Wolffian duct ; K, ccelom ; x, inner part of somatopleure; 

 y, inner part of splanchnopleure ; A, amniotic fold ; a, aorta ; e, hypoblast. IV, Scheme 

 of a longitudinal section of an early embryo. V, Scheme of the formation of the head- and 

 tail-folds r, head-fold ; D, anterior extremity of the future intestinal tract ; S, tail-fold, 

 first rudiment of the cavity of the rectum. VI, Scheme of a longitudinal section through 

 an embryo after the formation of the head- and tail-folds A o, omphalo-mesenteric arteries; 



