REPRODUCTION. 71 



an early stage. The ovum at first, in this case as in all others, 

 a single cell, becomes complex by addition of other cells (dis- 

 cus proligerus, etc.), which go to make up the yelk. All the 

 other parts of the hen's egg are additions made to it, as ex- 

 plained before, in its passage down the oviduct. The original 

 ovum, remains as the blastoderm, the segmentation of which 

 may now be described briefly, its character being obvious from 

 an examination of Fig. 68, which represents a surface view of 

 the segmenting fertilized ovum {oosperm). 



A segmentation cavity appears early, and is bounded above 

 by a single layer of epiblast cells and below by a single layer of 

 primitive hypoblast cells, which latter is soon composed of sev- 

 eral layers, while the segmentation cavity disappears. 



The blastoderm of an unincubated but fertilized egg consists 

 of a layer of epiblastic cells, and beneath this a mass of rounded 

 cells, arranged irregularly and lying loosely in the yelk, consti- 

 tuting the primitive hypoblast. After incubation for a couple 

 of hours, these cells become diflferentiated into a lower layer of 

 flattened cells (hypoblast), with mesoblastic cells scattered be- 



''^"^^m^^^m^ 



Pig. 69.— Portion of section througli an unincubated fowl's oosperm lafter Klein). 

 a, epiblast composed of a single layer of columnar cells; ft, irregularly disposed 

 lower layer cells of the primitive hypoblast; c, larger formative cells resting on 

 white yelk; f. archenteron. The segmentation cavity lies between a and J, and 

 Is nearly obliterated. 



tween the epiblast and hypoblast. It is noteworthy that, in the 

 bird, segmentation will proceed up to a certain stage independ- 

 ently of the advent of the male cell, apparently indicating a 

 tendency to parthenogenesis. 



The fowl's ovum then belongs to the class, a portion of which 

 alone segments and develops into the embryo (meroblastic), in 

 contradistinction to what happens in the mammalian ovum, the 

 whole of which undergoes division (holoblastic) ; a distinction 

 which is, however, superficial rather than fundamental, for in 

 reality in the fowl's egg the whole of the original ovum does 

 segment. This holoblastic character of the mammalian ovum 



