FROM LAYING TO FORMATION OF FIRST SOMITE 85 



end. A similar marginal notch at the hinder end of the blasto- 

 derm in the line of prolongation of the primitive streak has been 

 described also by His and Raiiber, but in the cases observed 

 by them there was no connection with the primitive groove. 

 It suggested to them, however, the idea of genetic connection 

 between the two, and was used as argument for the derivation 

 of the primitive streak from the margin by concrescence. 



The second question concerning the primitive streak, its role 

 in the formation of the embryo, may be answered very briefly 

 by saying that it is itself the primordium of the greater portion 

 of the axis of the embryo; some indeed maintain that it represents 

 the entire embryonic axis excepting the short pre-chordal part 

 (Kopsch). The view of Balfour and Dursy that it takes no essen- 

 tial part in the formation of the embryo, but atrophies as the 

 embryo forms, is now of historical interest only. The question 

 is how much of the embryo is represented by the primitive streak. 

 But this question is by no means easy to answer, and there is 

 no complete agreement in regard to it. The one point that is 

 definitelv settled is that the anus arises at the hinder end of the 

 primitive streak; but what point in the embryo corresponds to 

 the anterior end of the primitive streak, or, in other words, how 

 much of the embryo is laid down in the blastoderm in front of 

 the primitive streak, is a disputed question. The attempt has 

 been made to solve the problem by destroying the anterior end 

 of the primitive streak by a hot needle, or by electrolysis, then 

 sealing up the egg and permitting it to develop farther and finally 

 locating the resultant injury in the embryo. But, while one 

 worker finds the injury at the anterior end of the notochord 

 (Kopsch), that is in the region of the fore-brain, another finds it 

 in the region of the heart, that is in the hind-brain (Peebles). 

 The reasons for this discrepancy in results are two : (1) the methods 

 employed are not sufficiently exact, and (2) it is difficult in the 

 living egg to determine the exact location of the anterior end of 

 the primitive streak, and sometimes even to distinguish it from 

 the head-process. Owing to the extremely rapid growth of all 

 parts of the embryonic axis, a minute division of the primitive 

 streak becomes a relatively long part of the embryonic axis in a 

 very short time. It is obvious, therefore, that the slightest 

 deviation of the injury from the point aimed at may lead to 

 considerable error in the results. Until embryologists operate 



