397 



there are primitively two, for the mid-brain, and three, for the fore- 

 brain, requires modification. 



Squalus acanthias is, if we may judge from the primitive re- 

 lations exhibited by some of its organs, a very important form. 

 There are, for instance, in this animal three pairs of optic vesicles 

 serially arranged on the cephalic plate x ) and I have shown 2 ) that 

 the first pair form the lateral eyes and the second pair the pineal 

 outgrowth. In Squalus, the neuromeric segmentation, also, is very 

 distinct and makes its appearance very early. From the work of 

 previous observers it would be understood that these segments arise 

 after the neural groove is closed or while it is in the act of closing. 

 Waters carries the idea throughout his paper that the metameric 

 segmentation is relatively late, especially in the Teleosts, where he 

 was unable to find any trace of this segmentation earlier than the 

 sixth day of development. Orr and Mc Clure, do not in every case, 

 state ages but from their figures and the text I understand that they 

 have not detected this segmentation in any very young stage. The 

 earliest recorded observation with which I am acquainted is for the 

 thirty-hour chicken, but in all the figures I have seen illustrating 

 this segmentation the auditory vesicles are already formed. What- 

 ever may be the case in Teleosts and the lizard, I can say from my 

 own observations on Amblystoma (one of the forms observed by Orr, 

 Mo Clure and Waters) that the metameric segmentation is present, 

 just after the formation of the medullary folds, and during their 

 period of broadest expansion (see fig. 11). In Squalus acanthias, the 

 form I have especially studied, the neural segments are evident even 

 earlier than this. They may be seen, in the trunk region and 

 embryonic rim just after the embryo begins to be formed — in 

 Balfour's stages B and G. These are stages preceding the formation 

 of the medullary folds. 



II. Descriptions of the Stages examined. 

 The earliest stage in which I have detected the metameric 

 segmentation is represented in fig. 1. This is an age somewhere be- 

 tween Balfour's stages B and C 3 ). The axial part of the embryo 



1) Journ. Morph., Vol. IX, No. 1, Oct. 1893. 



2) Anat. Anz., IX. Bd., No. 5 und 6. 



3) I use the terms proposed by Balfour, as a ready means of 

 fixing, approximately, the age of certain specimens, but, as will appear 

 later, I do not think that his stages are adequate to represent the typical 

 stages in the development of Elasmobranchs. 



