18 Dr. W. Salensky on HcicheVs Gastrjea Theory. 



tlrely disappears) ; the inner one is developed into a so-called 

 8LS-hooked embryo, •which consists only of homogeneous cells. 

 We may certainly conn)are with a Plamtla that state of the 

 embryos of the Taniiv and Bothriocephalida^ in wiiich they 

 consist of a two-layered body (therefore before the development 

 of the embryo and the embryonal membrane). 



In the other animals which ])ass through the J/oru/a-stagc 

 the dirterentiation of the germ-lamella^ takes place in an exactly 

 similar manner as in the above-mentioned cases (someCopepoda, 

 some (Jammarida', probably the Ctenophora and the Caden- 

 terata, llydroid polypes, and Sponges). After the segmentation 

 the uniform cells divide into two layers, which represent two 

 germ-lamella', and become further developed into the organs. 

 Unfortunately, in the investigations of the development of 

 many of these animals, the question of the formation of the 

 germ-lamellai has been very little referred to. It appears to 

 me that in many instances the entoderm has been explained 

 as the nutritive vitellus. But until the formation of the intes- 

 tinal epithelium in the lower Crustacea has been further in- 

 vestigated, we may affirm with perfect justice, from the analogy 

 of the developmental processes in animals which have been 

 better investigated, that the central spherules, abounding in 

 fat, of the crustacean embryos really form the entoderm and 

 not the nutritive vitellus. That in many instances we can see 

 no cells in this part is due to its opacity. In Astacus Jluvia- 

 tilis the peripheral pai'ts of the cells of the entoderm, from 

 which the intestinal epithelium is formed, are also very diffi- 

 cult to observe, and only become distinct when they are tinged 

 with carmine or some other colomnng-material. At any rate 

 in this instance also we obtain, as the result of dift'erentiation, 

 the same temporary body-form, consisting of two layers, 

 and possessing no cavity in its interior — that is to say, the 

 Plamda. 



In some instances, in which we decidedly have the same 

 process before us, it may be obscured by certain subsidiary 

 phenomena. In most cases this masking is caused by the 

 occurrence of the nutritive vitellus, which is accumulated in the 

 Qg^ in larger or smaller quantities. Such cases occur, for ex- 

 ample, in the Cephalopoda, in Reptiles and Birds, and also in 

 Fishes. Here the egg-cell which becomes segmented is 

 situated at one pole of the ^gg. Tlie segmentation may be 

 com})ared to the regular segmentation, inasmuch as the cells 

 produced by the segmentation are at first uniform and subse- 

 quently differ from one another. It is only at a later period 

 that the differentiation of the germ-lamellae occurs in this 

 aggregation of cells ; the genn-lamellae are mutually arranged 



