364 Herr B. Lwoff on the Formation of the 



way that the raacromeres become inva^inated ; where the 

 blastula consists of several layers (as in Petromyzon and 

 Amphibia) the micromcres simply grow round the macro- 

 meres. Since these macromeres form the intestine, they may 

 with perfect justice be designated as endoderm cells ; the 

 micromeres, on the contrary, which differ noticeably from the 

 macromeres and give rise to the outer covering, are to be 

 termed ectoderm cells. I see no reason to distinguish an 

 ectodermal and an endodermal half after the first divisions of 

 the ovum have taken place, or it may be after the first equa- 

 torial furrow (as is the practice of certain investigators). 

 Such a distinction seems to me to be lacking in foundation at 

 this stage, since each supposed endoderm cell of this kind may 

 divide into two or more cells, of which one will subsequently 

 become an ectoderm cell by position, while the other will 

 belong to the endoderm. The distinction of the ectoderm 

 from the endoderm is only possible when the blastula is 

 already formed and the macromeres are being overgrown by 

 the micromeres. In this connexion it makes no difference 

 whether the macromeres are invaginated or are overgrown by 

 the smaller cells. I term them endoderm cells not on account 

 of invagination, but merely because they form the intestine. 

 But the process in consequence of which the endoderm cells 

 come to lie within and become surrounded by the cells of the 

 ectoderm can be homologized with that of gastrulation. 

 This process is to be regarded as preparatory to the formation 

 of the intestine. But besides this process of gastrulation, by 

 which the formation of the intestine is ushered in, there 

 appears on that side which subsequently becomes the dorsal 

 side of the animal another process, which represents the 

 invagination of the ectoderm cells, and which I will term the 

 dorsal invagination. This dorsal invagination is quite inde- 

 pendent of the gastrulation, and has nothing to do with the 

 formation of the intestine ; it gives rise to the common ecto- 

 blastogenous rudiment of the notochord and the mesoderm. 



This conception, which was mainly arrived at in the 

 investigation of the development of AmphioxuSj Petromyzon, 

 and Axolotl, I was also able to follow out in the interpretation 

 of the developmental processes of the Teleosteans and 

 Selachians, and it is moreover confirmed in the clearest 

 manner by the development of the Amniota. 



After these preliminary remarks I will proceed to the 

 account of my results. 



