20 Dr. W. Salcusky on HdcheVs Gastrsea Theory. 



secondary phenomenon, as we shall see hereafter. The stage 

 in which the tirst difterentiation of the gcnn-lanicUaj occurs is 

 of great importance, because it may serve for comparison with 

 the corresponding stages in other animals which also pass 

 through a Blastit/a-stagcm their ontogeny. By the occurrence 

 of tiie dirtorentiation the Bhu'ituhi will become equivalent to 

 the P/anula. To distinguish this stage from the true lilasfula, 

 which consists only of uniform cells, it may be called the 

 '' DibJastuIa'' (See PI. V. fig. 4). 



The same process of differentiation seems also to be very 

 common among Insects ; but it is in these somewhat masked 

 by the presence of a nutritive vitellus. But if we stick to the 

 principal characters of the case now expounded (namely, 1 , the 

 occurrence of a one-layered vesicle, and 2, the mode of diifer- 

 entiation of the germ-lamella^) , the first processes of insect- 

 development might also be explained in an analogous fashion. 

 These first processes are well known since the works of Zad- 

 dach, Weissmann, and Mecznikoff. The differentiation of the 

 germ-lamellas in Insects has been carefully investigated by 

 Kowalevsky. Kowalevsky, in his investigations, however, 

 has arrived at the conviction that the inferior germ-lamella 

 of Insects constitutes a peculiar formation, and cannot be com- 

 pared with that of the Vertebrata. lie compares the subse- 

 quently formed dorsal tube of Hydrophilus and the Phryganeidae 

 with the intestino-glandular lamella of the Vertebrata. I can 

 by no means share this opinion. To me the phenomena of 

 the formation of the germ-lamellae in the Ascidia appear to be 

 so in accordance with those of Hydrophilus^ that I certainly 

 can find no obstacle to regarding the inferior germ-lamella of 

 Hydrojjhilus as homologous with that of the Ascidia (and 

 therefore also oi Aviphioxus). We have seen that in the As- 

 cidia the differentiation of the inferior germ-lamella is effected 

 by some cells of the Blastula (the cells of the entoderm) be- 

 ginning to distinguish themselves from the others (the cells of 

 the exoderm). According to Kowalevsky 's researches this 

 differentiation in Uydrophilus commences in a perfectly similar 

 way. In order to make it easier to see our way with regard 

 to the homology of these two formations, I have given two 

 figures from Kowalevsky's memoirs (PL V. figs. 4 and 5). 



In the Ascidia a vesicle consisting of uniform cells is first 

 of all produced from the segmentation-cells ; in the Insects 

 also the same vesicle occurs, differing from the former only 

 by its being filled with vitellus. In the Ascidia the differen- 

 tiation of the germ-lamella3 is brought about by some cells of 

 this vesicle beginning to distinguish themselves from the 

 others, and thus forming the foundation of the inferior germ- 



