26 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



blast. The superficial epiblast, again, is formed of flattened cells, which 

 soon become columnar and appear to unite with the rounded elements 

 below, except at the lower part of the embryonic area. Here the blasto- 



FIG. 16. RABBIT'S OVUM BETWEEN SEVENTY TO NINETY HOURS AFTER 

 IMPREGNATION, AFTER E. .VAN BENEDEN. (Sal/our.) 



BV, cavity of blastodermic vesicle (yelk-sac) ; EP, epiblast ; H Y, hypoblast ; ZP, mucous envelope 

 (Zona pellucida). 



derm, as in the chick, is constituted by three layers, the epiblast, the 

 mesoblast, and the hypoblast. 



P.R, 



EP. 



M 



HY. 



FIG. 17. SECTION THROUGH THE OVAL BLASTODERM OF A RABBIT IN THE 

 SEVENTH DAY, THROUGH THE FRONT PART OF THE PRIMITIVE STREAK. 

 (Balfour.) 



EP, epiblast; M, mesoblast; HY, hypoblast; PR, primitive streak. 



The subsequent changes in the development of the blastoderm form 

 the subject of Embryology, and for their consideration the reader is 

 referred to text-books on anatomy. 



III. THE MODIFICATION IN THE FOKM OF CELLS. 



We have seen that originally all the cells formed by cleavage in the 

 egg are absolutely alike. Like the original egg, they are typical cells, 

 consisting of a cell-membrane inclosing finely granular protoplasm, in 

 which a nucleus and nucleolus may be recognized. They only differ from 



