1881.] 



and Early Development of the Mole. 



195 



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 millim. 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 primitive streak ; it is narrow an- 

 teriorly, while posteriorly it becomes broader, and finally behind takes 

 up nearly the whole 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 embryonic 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 

 nenrenteric 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 millim., the primitive streak reaches 

 along it fully three parts of its length. It is very narrow in front, 

 while behind it occupies the whole breadth of the embryonic area. 

 In sections of the region in front of the primitive streak there is 

 present a layer of cells several rows thick immediately underlying the 

 epiblast plate. In the seven anterior sections this layer is seen 

 beneath the epiblast, as amass which cannot be resolved into hypoblast 

 and mesoblast ; for about three following sections, placed immediately 

 in front of the primitive streak, the layer is clearly composed of (1) a 

 layer of flattened hypoblast below, and (2) a layer of mesoblast above. 

 The mesoblast in the axial line is thickened in the last two of these 



