372 Heir B. Lwoff o/t the Formation of the 



In the formation of tliesc parts of the mesoderm, which differ 

 only in position, tlic cell.s of both ectoderm and endoderm 

 participate. These two sources of the mesoderm cells are very 

 distinctly visible ; but the detailed demonstration must be 

 reserved for my subsequent paper. I will here but briefly 

 allude to one more point. It is evident from what has been 

 stated that I by no means re:^ard the Selachians as ])rimitive 

 forms, from which other animals are to be derived. I see no 

 reason to derive the Amphibians from the Selachians, as is 

 done by certain investiorators. This amounts to deriving 

 simple and primitive conditions from such as are complicated 

 and modilied. Although the Selachians in many respects 

 exhibit primitive characteristics, their conditions are never- 

 theless so much modified in consequence of the superabun- 

 dance of the food-yolk, that they can be regarded as the 

 ancestral type of other animals just as little as the Teleostean 

 fishes. 



In the case of Lacerta no difficulty is experienced in distin- 

 guishing the two primary germinal layers, since we here get 

 a bilaminate germinal disk as the result of segmentation. At 

 the posterior margin of the germinal disk we may observe an 

 active multiplication, and in consequence of this a prolifera- 

 tion of the ectoderm cells. This proliferating spot in the 

 ectoderm, which in the case of Amniota is termed the primitive 

 tubercle (" Primitivknopf "), primitive plate, primitive streak, 

 and I know not what besides, gives rise to the ectoblasto- 

 genous rudiment of the notochord and mesoderm, which, 

 starting from this point in a forward direction, grows in 

 between the two primary germinal layers as a continuous 

 plate of cells. In the middle of the primitive streak, but 

 nearer its anterior end, there may be noticed a depression by 

 which the invagination is ushered in. In this manner there 

 arises the margin of the enveloping layer at which tlie cells of 

 the rudiment of the nervous system and those of the rudiment 

 of the jiotochord bend round into each other, and where nume- 

 rous mitoses are to be seen. The direction of these mitoses 

 shows in the clearest manner that the cells of the rudiment of 

 the notochord grow forward from the margin of the over- 

 growth. This invagination in no way leads to the formation 

 of the intestine, since all the invaginatcd cells are absorbed in 

 the production of the notochord and mesoderm ; but the 

 intestine arises from the cells of the lower primary germinal 

 layer. I have no reason in this case to designate the process 



