34 M. Pli. Owsjannlkow on the 



grown longer and has become pointed at one end. This latter 

 appearance is due to the fact that during this period the 

 multiplication of cells proceeds with especial activity at the 

 upper lip of the anus of Rusconi at the point where the upper 

 layer passes into the lower. 



Since during this period — it may be some hours earlier, 

 though usually later — many highly interesting processes take 

 place in the ovum, we will now consider these more closely. 



The Foemation of the Alimentary Canal. 



In earlier stages the anus of Rusconi took the shape of a 

 broad pit which had arisen by invagination of the epiblast. 

 I am speaking of an embryo which is at least some four-and- 

 twenty hours younger than that represented in Scott's 

 fig. 10, a. At that period we actually have a gastrula before 

 us. In a few hours, however, the number of the yolk-spheres 

 lying at the bottom of the ovum has considerably increased. 

 The diminution in the size of Baer^s cavity during this time 

 is not to be ascribed to the fact that the cells are thrust into 

 it by the formation of the enteric cleft, but to the increase in 

 the number of these cells. 



The formation of the enteric cavity proceeds by a splitting 

 off of the vitelline elements from the undifferentiated cells, 

 which become the hypoblast ("von dem Driisenkeim "), 

 starting from Rusconi's pit, precisely as this process has often 

 been observed and described in the frog, axolotl, and sterlet. 

 In the animals just mentioned this process is easier to observe, 

 because the cells which adjoin the cleft contain pigment- 

 granules duringtheir division and separation from the remainder 

 of the yolk. In some cases a streak of pigment precedes the 

 cleft. Since the ova of Petromyzon are entirely unpig- 

 mented, observation becomes somewhat more difficult. In 

 spite of this we can convince ourselves by the examination of 

 hundreds of preparations that the lower layer, the endoderm, 

 arises by the separation of a series of cells from the yolk. It 

 is further to be remarked that before the formation of the 

 enteric cleft the sections already show a special grouping of 

 the cells which are subsequently utilized as the elements of 

 the lower layer. During this period and also somewhat later 

 two layers, the ectoderm and endoderm, are present, which are 

 separate from one another and continuous only at the point of 

 flexure. The cells of the ectoderm are smaller than those of 

 the endoderm, as is indeed perfectly natural, since the deve- 

 lopment of the former commenced earlier than that of the 

 latter. 



