112 WALTER HEAPE. 



prises the portion of the wall opposite to the embryonic area> 

 while the former, consisting of epiblast and hypoblast, forms 

 the embryonic area and the part of the vesicle immediately 

 adjoining it. 



In the course of further growth the vesicle greatly enlarges, 

 and the zona becomes much attenuated, affording but little 

 support to the now also exceedingly thin and delicate wall of 

 the vesicle ; it therefore becomes difficult to obtain specimens 

 in good preservation. 



In the earliest specimen of this stage which I possess the 

 embryonic area is oval, measuring '74 by '48 mm. In a 

 surface view a dark line or band is seen to run along the centre 

 of the hinder third of the area. This is the well-known primi- 

 tive streak ; it is narrow anteriorly, while posteriorly it 

 becomes broader, and finally behind takes up nearly the 

 whale breadth of the embryonic area; it is due to the 

 presence of a third layer of cells, the mesoblast, between the 

 epiblast and hypoblast. Transverse sections of this em- 

 bryonic area show the major portion in front of the primitive 

 streak to be composed (1) of a plate of epiblast formed of 

 two or three rows of columnar cells, and (2) of a single layer 

 of rounded hypoblast cells somewhat flattened towards the 

 edge of the area. Immediately in front of the primitive 

 streak there appears, extending entirely across the area, a 

 layer of mesoblast, which is not connected with the epiblast, 

 but is so intimately united with the hypoblast in the middle 

 line, that the two layers cannot there be clearly distinguished, 

 though towards the periphery of the area they are quite 

 distinct. A section taken through the anterior end of the 

 primitive streak discovers a narrow band of epiblast cells in 

 the middle line, giving rise by budding to a layer of mesoblast 

 which extends laterally to the edge of the area, and in each 

 section following {i.e. towards the hind end of the primitive 

 streak) the budding epiblast appears continually as a wider 

 band until the greater part of the whole breadth of the 

 epiblast plate is concerned in the production of mesoblast. 

 A pit is seen in the epiblast almost at the front end of the 

 primitive streak, and at this point a neurenteric canal will 

 eventually be formed ; this structure, hitherto overlooked in 

 mammalian embryos, is identical with the neurenteric canal 

 found in other types of Vertebrata. 



The primitive streak grows relatively longer compared 

 with the increase in size of the embryonic area, until in a 

 vesicle, in which the latter measures about "84 by '71 mm., 

 the primitive streak reaches along it fully three parts of its 

 length. It is very narrow in front, while behind it occupies 



