5^8 LOCV. [Vol. XI. 



the mesoblast, but it is not clear in my mind that we are justi- 

 fied in looking upon the divisions as primitive mesoblastic 

 somites ; before seeing Froriep's paper I had considered them 

 as undulations, probably due to the same influence that has 

 thrown the epiblast so clearly into segments. In many 

 embryos of Squalus, showing epiblastic segmentations, they 

 are, so far as I can determine from sections, entirely lacking. 

 It seems to me that they are rather to be looked upon as a 

 consequence of primitive segmentation and not as protoverte- 

 bras. This view is substantiated by the fact that the formation 

 of the latter takes place in the usual manner at a later period. 



Considering the late stages in which the neuromeres have 

 generally been described, it was a natural inference that they 

 arise after the neural tube is established. Minot, in expressing 

 his conception of the formation of the neural segments, based 

 on the descriptions of Orr and McClure, says : " Their appear- 

 ance seems to depend on the development of the primitive 

 segments of the mesothelium. When the segments are fully 

 formed, and before their inner wall has changed into mesen- 

 chymal tissue, they press against the medullary tube and 

 oppose its enlargement ; at least one sees that the tube 

 becomes slightly constricted between each pair of segments 

 and slightly enlarged opposite each inter-segmental space." 

 But the neural segments appear so much earlier than the 

 primitive segments of the mesothelium, that this interpreta- 

 tion is no longer tenable. 



The facts made known will tend to materially modify the 

 current theory of metamerism, which assumes as fundamental 

 that metameric divisions begin in and depend on the mesoblast. 

 As Adam Sedgwick says, in a recent paper : " The segmen- 

 tation which obviously persists in the trunk region, and which 

 begins with segmentation of the mesoderm, and is moulded 

 upon it in the manner characteristic of all metamerically seg- 

 mented animals." It seems to me a strained inference, that 

 the middle layer — the last germ-layer to be formed — should 

 be the bearer of this primitive metameric division, but, of 

 course, the final appeal must be made to sections and my 

 sectioned material has given evidence entirely corroborative of 



