590 ON THE EARLY STAGES IN THE DEVELOPMENT OF THE EMU, 



A comparison of this blastoderm with the preceding one, and a 

 comparison of corresponding stages in the fowl will show that the 

 notochord extends backwards much more rapidly than the whole 

 embryonic region ^^^tts the primitive-streak region increases in 

 size. This it can only do at the expense of the cells of the axial 

 plate, with which it is continuous behind. Since, however, the 

 notochord is never found to extend backwards in this axial plate, 

 it follows that as the former grows backwards the cells of the 

 latter become detached from the epiblast and spread out, so as to 

 resemble in their arrangement the mesoblast cells in front of them. 

 There is in this way a progressive separation from before back- 

 wards of the deeper part of the axial plate from a surface layer of 

 epiblast. Thus, in a sense, the primitive streak takes part in the 

 formation of the hinder part of the embryo, becoming at the same 

 time gradually reduced, till it occupies at last only an extremely 

 small space at the posterior end of the embryo. In this manner 

 the anterior part of the primitive streak becomes the pos- 

 terior part of the medullary plate, and the primitive 

 groove in its anterior part is not separate from the medul- 

 lary groove, and really becomes converted into the posterior 

 part of the latter. This will account for the great length of the 

 primitive streak ; it does not entirely represent the coalesced lips 

 of the blastopore, but the anterior part is the foundation of the 

 embryonic area. 



This is substantially the same as Kolliker's* account of the 

 origin and history of the notochord and the destiny of the primi- 

 tive streak in the chick ; and Braunf gives a similar account 

 for the Melopsittacus. It is not, however, that given by the 

 majority of embryologists, who differ greatly not only as to the 

 mode of formation of the chorda, but as to its subsequent mode 

 of o-rowth, and the share which the primitive streak has in further 

 development. J 



* XXIV. 

 + X. 



X See the memoirs of Balfour, HoiTmami, and Gerlach cited below. _ For 

 a summary of opinions as to the part taken by the primitive streak in the 

 formation of the embryo, see Kolliker, XXIV, pp. 134-138. 



