42 DEAN. [Vol. XL 



if not a large part, of blastopore is retained as the hindermost 

 portion of the neurenteric canal, its greatly diminished yolk 

 contents disappearing. In the surface view, PI. Ill, Fig. 54, 

 may be seen prominently the neural canal, primitive segments, 

 and, now depressed and passing under the flattened tail mass, 

 the pronephric ducts. 



The origin and fate of the vesicle of Kupffer appears closely 

 connected with the closing blastopore. Within the entire 

 circle of the blastopore's rim occurs the trench which in sec- 

 tion has been noted in the different stages of gastrulation. 

 When seen in vertical section it appears under the dorsal lip 

 as the deep recessus which the present writer regards as 

 Kupffer's vesicle. In Sturgeon the fate of this structure 

 (which may be followed in the sections, PI. IV, Figs. 64, 65, 

 66y 6Z, 69) shows conclusively that it can be regarded as but a 

 growth adaptation of the gastrula, a condition due to an extreme 

 thickening of Randwulst. It is most marked in the mediati 

 line in the tail region, since it is here that the embryo has 

 acquired a keel-like thickening which exerts a mechanical in- 

 fluence in deepening the Randwulst and causing the underlying 

 cavity to become enlarged. In the Teleost where the concen- 

 tration of the embryo in the median plane is extremely marked, 

 Kupffer's vesicle may well represent an expression of its mode 

 of growth : the germ ring is clearly the rim of the blastopore 

 — or more accurately perhaps the circumcrescence margin — of 

 Acipenser. This interpretation of the vesicle, the writer be- 

 lieves, would adequately explain its peculiar conditions in 

 Salino or Esox : in Salnio fario he has examined the vesicle 

 and verifies the observation of Henneguy as to its possessing 

 a flooring of entoderm cells : this layer, loosely separate from 

 the undifferentiated cells in the anterior region of the tail mass, 

 might, the present writer believes, readily remain in contact 

 with the periblast below, as the mechanical growth change 

 caused the vesicle to appear. 



The outline of the early development of the Sturgeon might 

 now best be completed by a discussion of the differentiation of 

 the germ layers. 



The origin of the outer layer has already been traced (pp. 



