REPRODUCTION. 



73 



The first indications of future structural outlines in the em- 

 bryo is the formation of the primitive streak, an opaque band 



Tie. 71.— Diagrammatic transrerae eections thioagli a hypothetical mammal oOsperm 

 (Haddon). A. The yelk of the primitive mammalian oosperm is now lost. B. 

 Later gtage; the non-embryonic epiblast has grown over the embryonic- area to 

 form the covering cells; ep, epiblast of embryo; ep', epiblaet of yells-sac; hy, 

 primitive hypoblast; y. s, yelk-sac, or blastodermic vesicle. 



in the long diameter of the pellucid area, opaque in consequence 

 of cell accumulation in that region. Very soon a groove (primi- 

 tive groove) extends through- 

 out this band, which gradu- 

 ally occupies a more central 

 position. The relative thick- 

 ness of the several parts and ji 

 the arrangement of cells may 

 be gathered from Fig 73. 

 These structures are only 

 temporary, and those that re- 

 place them will be described 

 subsequently. 



We have thus far spoken 

 of cells as being arranged in- 

 to epiblast, hypoblast, and 

 mesoblast. The origin of the 

 first two has been sufficiently 

 indicated. The mesoblast 

 forms the intermediate ger- 

 miual layer, and is derived 

 from the primitive hypoblast, 

 which differentiates into a 



stratum of flattened cells, Fib. 72.-Sarface view of peUncid area of 



situated below the others, Wastoderm of eighteen homs (Foster and 



J i-.i_ i- ,,i' , J Balfonr). B, medullary folds ; mc, me- 



and constituting the later dnllary groove; pr, primitive groove. 



