ANIMAL BIOLOGY. 



[Part I. 



illustrations'!!! this figure exhibit the relations of epiblast, hypo- 

 blast, and mesoblast in the foetal membranes. 



Meanwhile beneath the embryo the yolk-sac, surrounded 

 by a thin blastodermic layer, is in contact with the investing 

 membrane. But by the continuation of the cleavage into this 

 region the blastoderm is split into two layers, of which one 

 (continuous with the splanchnic layer) remains adhering to the 

 yolk-sac, while the other (continuous with the somatic layer 

 outside its amnion fold) adheres to the investing membrane, and 

 forms the serous membrane of this region. This will, perhaps, 

 be sufficiently clear by a study of iii. and iv., Fig. 38. Note 



tjro.am. 



FIG. 38 A. RELATIONS OF EPIBLAST, HYPOBLAST, AND MESOBLAST. 

 I. Transverse section of Embryo before the Amnion folds have met. 

 II. Transverse section of Embryo (Amnion omitted) through Pharynx and 



Heart- folds. 

 III. Longitudinal section of Babbit Embryo to show Proamnion. 



Thin line, epiblast ; thick line, hypoblast ; dotted, mesoblast. 

 Allantois. am. Amnion. am. f. Amnion fold. cod. Crelom. 7t. /. 

 Placental area. so. Somatopleure. sp. 

 I mesoblast. 



Heart fold. ph. Pharynx, pi. a. Pla 

 Splanchnopleure. u. m. Uncleaved me 



first that in iii. the whole of the amnion fold down to x is con- 

 tinuous with the somatic layer (so. L). Then note that in iv. the 

 serous membrane, derived from the outer layer of the amnion 

 fold, forms a distinct membrane as far as x. Now, picture the 



