S6 ^r. Vh. Owsjannikow on the 



a single layer. A section taken through the anterior portion 

 of llic embryo presents anotlicr picture. At this point the 

 central nervous system is tliickcr : a central canal has already 

 developed at some distance from the brain, the anterior end of 

 which is also still solid. At this region the notochord is not 

 present, though the cells of the endoblast immediately adjoin 

 the central nervous system. At some distance from the ante- 

 rior end of the embryo the enteric cavity is of a very con- 

 siderable size. It is surrounded above by a double layer of 

 endoblast cells, whicli are somewhat thrust downwards in the 

 middle by the spinal cord. On the other hand, on both sides 

 of this depression, to the right and left of it, tlie endoblast 

 forms an upward expansion of the cavity. If we now 

 examine the sections in order from beliind forwards, we 

 observe two folds of the endoblast approaching nearer and 

 nearer to the middle line. In this manner two cavities 

 appear, the uppermost of which is the smaller. Finally the 

 folds completely meet, forming a ring round the smaller 

 cavity, which at last entirely disappears. 



In the remainder of the body tlie formation of the notochord 

 appears to take place by a simple constricting off of the 

 endoblast cells lying beneath the spinal cord, as has already 

 been described and figured by Calberla. On both sides of 

 the notochord lies a double row of cells, which soon passes 

 into a single one. These cells have separated from the 

 endoblast. There is no space between the two rows ; but in 

 spite of this we already recognize the elements which will go 

 to form protovertebra?, Wolffian ducts, and lateral plates. 

 Moreover in many sections we find the protovertebra already 

 almost completely developed : the formation of a cavity in 

 them takes place later on. The cpiblast, too, during this 

 period is still exclusively unilaminate except in the median 

 line, where the spinal cord has already developed. In the 

 sections it is still sometimes entirely separate from the ovum. 

 The development of all primitive organs is further advanced 

 in the posterior than in the anterior portion of the embryo. 

 The formation of the cavities and canals is effected either by 

 the separation of the cells from one another or, as is especially 

 the case in the region of the undifierentiated cells which 

 become the iiypoblast, by the consumption of the yolk-lamellaj, 

 80 that in the place of the yolk-ceils only emi>ly envelopes 

 remain. 



^ Development proceeds from the median line towards the 

 sidcR. After the formation of the spinal cord and the noto- 

 chord a cavity arises, first in the protovertebra;, which separate 

 from the rest of the cells of the mesoblast, then in the Wolffian 



