GERMINAL LAYERS AND EARLY DEVELOPMENT OF MOLE. 115 



thick than the remainder of the epiblast plate, but in the 

 deeper part of the groove become considerably 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 specimen. 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, however, where the lateral mesoblast is commencing to 

 form protovertebrse, the notochordal cells, still in the form 

 of a single row, are connected solely with the lateral 

 plates of hypoblast, while further back, where the medullary 

 groove becomes again more shallow, the cells of the noto- 

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

 tinuous both with the lateral mesoblast and hypoblast. The 

 notochord, continually thickening towards its hinder ex- 

 tremity, terminates by fusing with the anterior wall of the 

 neurenteric canal. The latter structure is now open above 

 on the floor of the posterior end of the medullary groove, 

 and extends downwards into the cells beneath, though it no 

 longer perforates the hypoblast, which is, however, somewhat 

 involuted, and indistinguishably fused with the mesoblast in 

 the median line. 



Briefly to recapitulate, I have attempted to show : 



(1.) The epiblast of the blastodermic vesicle owes its 

 origin as well to the inner mass of segmentation spheres as 

 to the outer layer of segments. It appears to originate in 

 two ways : 



(a.) In an early stage of development (in the mole) 

 probably by the cells of the inner mass being directly 

 transformed into part of the wall of the blastodermic vesicle. 



{b.) In a later stage (mole and rabbit) by the transforma- 

 tion of the rounded cells of the inner mass into a plate of 

 columnar cells, which joins the part of the outer layer lying 

 immediately above it to form the epiblast plate of the 

 embryonic area. 



(2.) The mesoblast in the mole is formed in two portions : 



(a.) A larger portion which has its origin in the primitive 

 streak. 



(b.) A smaller portion which is derived from the hypoblast 

 situated in front of the primitive streak. 



