26 DEAN. [Vol. XL 



layers of the middle layer which form them are early separate, 

 and as traces of a lumen in early segments are to be found. 

 The solid character of the later muscle plates appears strikingly 

 Teleost-like. These the writer believes furnished the basis of 

 Beard's observations ; they certainly appear, however, long 

 before traces of the auditory capsules appear, and rather in the 

 mid-region of the embryo's axis ; their increase is then as Beard 

 states at the expense of the hinder tissue. 



The definite origin of the central nervous system does not 

 appear in a stage much earlier than that of PI. I, Fig. 19, and it is 

 here only in the anterior region of the embryo that its relations 

 may be determined. In the hinder trunk region the formative 

 epiblast is found to be generally thickened but otherwise un- 

 differentiated in the median line ; in the mid-region its layer 

 becomes thinner and flatter ; in the head eminence, on the 

 contrary, it becomes thicker, its boundaries are well marked, 

 and it is deeply implanted in the median line ; a median cell 

 cord is now to be recognized as its outward expression ; as yet 

 a lumen has not appeared and the general appearance of the 

 spinal cord is teleostean : no sense organs have as yet made 

 their appearance. The prominent margins of the head eminence 

 are formed of the anterior extension of the mesoblast ; and the 

 writer believes that it was this mesoblastic rim of the head 

 eminence that Balfour and Parker have figured as the early brain 

 vesicles. The writer notes that the anterior end of the central 

 nervous system in this stage ends not in a point of contact with 

 the cells of the ectoderm (lobus olfactorius impar^) but in a thin 

 continuous cell sheet which underlies the ectoderm and forms 

 the roof of the vascular area (segmentation cavity). The rela- 

 tion of the mesenchyme elements, which were earlier seen- 

 occupying the vascular area, to the formation of blood and 

 vessels is by no means easy to determine ; and the wide diver- 

 gence among observers in their studies on the origin of the 

 vascular system in well studied types seems to render inadvis- 

 able what can be but an imperfect note on the conditions of 

 Lepidosteus. The writer would record, however, that as far 



1 As Kupffer has demonstrated in the Sturgeon, and as others have recently 

 described in other vertebrates. 



