500 A. A. W. HUBREOHT. 



Before entering upon a description of the embryological 

 data, such as we notice them in the shrew, a short sum- 

 mary of the principal points upon which the above-named 

 authors agree, and of those upon which they dififer, may serve 

 to clear the ground, to refresh the memory, and to facilitate 

 the interpretation of the phenomena which are here brought 

 to bear upon the points in contest. 



In this brief summary I will aim at the utmost conciseness, 

 referring the reader who wishes to make a full study of the 

 intricate subject to the original treatises, of which I will give 

 a list in an appendix to this article. 



Omnium consensu, the primitive streak and the primi- 

 tive groove are looked upon in the light in which Balfour first 

 taught us to see them, i. e. as the region that corresponds to 

 the blastopore of Amphioxus^ of the Cyclostomata, and of the 

 Amphibia. 



The lips of the blastopore are stretched, and of the circular 

 opening a longitudinal groove (Primitiv-rinne) is the repre- 

 sentative, which may henceforth be designated, with Bonnet 

 and Fleischmann, as the gastrula-groove. 



The gastrula ridge (Gastrula-leiste, Fleischmann) is a very 

 adequate name for the tissue which proliferates inwards from 

 the epiblast in this region. This proliferating ridge was 

 hitherto designated as the primitive streak. 



From its cell-strata mesoblastic tissue spreads between the 

 two primary germinal layers. 



The inevitable consequence of our comparison with lower 

 types is that we have to look upon this median streak 

 of tissue that has proliferated inwards — although it un- 

 deniably originates out of the epiblast — as essentially a 

 formation of hypoblastic material (Urentoderm, Kupffer, 

 Bonnet, a. o.).^ 



• Tbia allows of considerable simplificatiou of one point, about which much 

 unnecessary controversy has been going on, viz. in how far the mesoblust of 

 the Amniota is partly of epiblastic, partly of hypoblastic origin. As soon as 

 the gastrula ridge is looked upon as representing archaic hypoblast, the 

 question as above formulated can advantageously be allowed to drop. 



