STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 743 



Development of the embryo. We have not been able to work 

 out the stages immediately following the segmentation, owing to 

 want of material ; and in the next stage satisfactorily observed, 

 on the third day after impregnation, the body of the embryo is 

 distinctly differentiated. The lower pole of the ovum is then 

 formed of a mass in which no traces of the previous segments or 

 segmentation furrows could any longer be detected. 



Some of the dates of the specimens sent to us appear to have 

 been transposed ; so that our statements as to ages must only be 

 taken as approximately correct. 



TJiird day after impregnation. In this stage the embryo is 

 about 3'5 millims. in length, and has a somewhat dumb-bell shaped 

 outline (Plate 34, fig. 5). It consists of (i) an outer area (/. z) 

 with some resemblance to the area pellucida of the Avian 

 embryo, forming the parietal part of the body ; and (2) a central 

 portion consisting of the vertebral and medullary plates and the 

 axial portions of the embryo. In hardened specimens the 

 peripheral part forms a shallow depression surrounding the 

 central part of the embryo. 



The central part constitutes a somewhat prominent ridge, the 

 axial part of it being the medullary plate. Along the anterior 

 half of this part a dark line could be observed in all our speci- 

 mens, which we at first imagined to be caused by a shallow groove. 

 We have, however, failed to find in our sections a groove in this 

 situation except in a single instance (Plate 35, fig. 20, x), and are 

 inclined to attribute the appearance above-mentioned to the 

 presence of somewhat irregular ridges of the outer layer of the 

 epiblast, which have probably been artificially produced in the 

 process of hardening. 



The anterior end of the central part is slightly dilated to form 

 the brain (b.) ; and there is present a pair of lateral swellings 

 near the anterior end of the brain which we believe to be the 

 commencing optic vesicles. We could not trace any other clear 

 indications of the differentiation of the brain into distinct lobes. 



At the hinder end of the central part of the embryo a very 

 distinct dilatation may also be observed, which is probably homo- 

 logous with the tail swelling of Teleostei. Its structure is more 

 particularly dealt with in the description of our sections of this 

 stage. 



