738 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



plates can be distinguished, for all these melt into a common aggregation of cells, below 

 which even the hypoblast, as Balfour and Deighton note also in the chick (No. 19, 

 p. 180), is hardly separable as a denned layer (PI. III. figs. 3 and 12, and PL IV. figs. 5b 

 and 5c). The epiblast (ep, PI. IV. fig. bd) laterally is partially differentiated ; but in 

 the middle line it merges in the cells below, to which, indeed, it gives origin. All these 

 features point to its identity with the primitive streak of higher forms. The primitive 

 streak, it is true, according to the accepted interpretation, arose in the process by which 

 the embryonic trunk, notably in the Sauropsida, was removed from a marginal to a more 

 central position on the surface of the yolk. This transference drew after the embryo, as 

 it were, the diverging arms of the blastoporic lip, and their cells form a post-embryonal 

 mass, which is the primitive streak. In Rana temporaria, as Spencer found (No. 151, 

 p. 97), the point where the medullary groove opens into the blastopore becomes solid, the 

 neurochord losing its canal, and the epiblast, mesoblast, and hypoblast fusing as an indif- 

 ferent mass just anterior to the blastopore. The Teleostean embryo reaches to the 

 periphery of the blastodermic area, and any similar aggregation of indifferent cells is 

 reduced to its smallest limit, yet such an aggregation exists, as a transient posterior 

 mass, into which the notochord and other structures, anteriorly placed, pass and disappear. 

 It is so in the chick, and in both the structure is transient — its importance goes with 

 the earliest embryonic stages, and it disappears, or rather is used up, partially as we have 

 seen, in the production of mesoblast, and still more by the extension posteriorly of the 

 embryonic trunk, and the development of the tail. Its position on the anterior margin 

 of the blastopore is easily explained, the present anterior margin is really the primitive 

 posterior margin. If the blastopore extended to the ventral surface of the embryo, an 

 increase in the amount of food-yolk would cause its true anterior margin to be pushed 

 away from the ventral surface, and as it was thus carried outwards, the true posterior 

 margin remaining unmoved, the parts of the blastopore would become reversed, just as a 

 pendulum, if held horizontally in a north and south direction with the weight north, would 

 with the first swing become reversed, the fixed attachment would point north, and the 

 weight (i.e., the true north end) would become south, and thus it is that the present 

 posterior edge of the blastopore is really the former anterior margin. According to 

 this view, we see that the blastopore, having drifted outwards, no longer coincides with a 

 ventrally placed anus ; and the relations of the primitive mesenteron, post-anal gut 

 (Kupffer's vesicle), and neurenteric connection with the dorsal groove are placed in a 

 clear light by means of the blastopore. 



