1881.] 



and Early Development of the Mole. 



197 



out, and the cells forming it are not more than two rows deep, while 

 the remainder of the epiblast plate, except at the extreme edge, is 

 three cells deep. 



Anterior to the medullary groove a continuous layer hardly differ- 

 entiated into mesoblast and hypoblast underlies the epiblast plate. 

 In the region of the medullary groove an axial strip of the cells 

 underlying the epiblast exhibits no division into hypoblast and meso- 

 blast; this portion, though partially separated from the lateral masses 

 of mesoblast, is still connected with them however on each side by a 

 narrow neck of cells, and is also directly continuous laterally with the 

 hypoblast. The lateral hypoblast is quite distinct from the super- 

 jacent lateral masses of mesoblast. The axial strip of cells under- 

 lying the medullary groove (thus shown to be continuous with both 

 the mesoblast and the hypoblast) may be regarded as the commencing 

 notochord. 



The neurenteric canal does not any longer perforate the blastoderm, 

 its upper part alone remaining, which is surrounded by a thick mass 

 of mesoblast, causing the dark rim seen in the surface view round the 

 pit. Its anterior wall is connected with the axial mass of cells under- 

 lying the medullary groove, while its hind wall forms the front end of 

 the primitive streak. 



The surface view of a somewhat older specimen, 1*5 millims. by *81 

 millim. diameter, shows the medullary groove relatively much longer 

 and more clearly defined. Anteriorly it reaches near to the edge of 

 the embryonic area. 



In section it is seen to be shallow at each end, but is much deeper 

 and narrower towards the middle of the embryonic area. Its walls, 

 at the anterior end, are but slightly less thick than the remainder of 

 the epiblast plate, but in the deeper part of the groove become con- 

 siderably thinner. Where the groove is deepest the notochord and 

 adjacent parts form a well-marked projection into the blastodermic 

 cavity beneath. 



The rudimentary notochord has now extended beyond the anterior 

 end of the medullary groove, and its relations to the adjacent layers 

 are, for the most part, the same as in the previously described speci- 

 men. In front of the medullary groove it is composed of a single row 

 of somewhat columnar cells, continuous laterally with both mesoblast 

 and hypoblast ; below the anterior more shallow part of the medullary 

 groove similar relations exist, towards its middle and deeper part how- 

 ever, where the lateral mesoblast is commencing to form protovertebrae, 

 the notochordal cells, still in the form of a single row, are connected 

 solely with the lateral plates of hypoblast, while further backwards, 

 where the medullary groove becomes again more shallow, the cells of 

 the notochord become more than one row deep, and are again con- 

 tinuous both with the lateral mesoblast and hypoblast. The notochord, 



