24 Dr. W. SSaleusky on HckkiTs Gastra-a Tluori/. 



TrocJuuij Vermetus, J-Jntoconcha, Sec.) takes place in the same 

 lUiUiiier. . 



The further developmental phenomena of tlie animals which 

 pa:?s through the /{Idstula-MU'j^c in the course of their ontogeny, 

 may occur in different ways. If we commence with the 

 erabn'onal state of the Ascidia, which constitutes a flattened 

 vesicle (PI. V. fig. 4), and in which the differentiation into 

 two germ-lamella' has already been eflfected, we see that the 

 subsetiuent phenomena consist in the whole embryo acouiring 

 a cup-like form (fig. o). This cup, consisting of two layers, 

 afterwards passes into the 6^«.s7rM/a-stage (as is well known in 

 the Asc'idm, A mjJu'oxus, Liuahricus, &c.). In consequence 

 of these changes (of the invagination) the stomachal cavity 

 of the Gastrula is produced ; but the stomach-wall has been 

 difi\ lentiated earlier, during the fiattening. 



AViiilst in tiie last-mentioned cases the embryo {Dihlastula) 

 is converted into the Gastrula-iorm, the corresponding Dihlas- 

 tula-ionn of the insect undergoes quite different changes. In 

 these the entoderm sinks into the nutritive vitellus, and is 

 gradually covered from without by the exoderm. The diver- 

 gence of the two corresponding stages of development in the 

 Ascidia and in Hydrojihilus, hoi\\ of which maybe derived 

 from a common Dihlastida-foni), is elucidated by the two 

 figures 6 and 7, in Plate V.* 



These differences in development lead finally to the totally 

 divergent conditions of the subsequent embiyonal phenomena 

 in these two animals. Whilst in the Gastrula (Ascidia) the 

 intestinal cavity is akeady sketched out, it will only be formed 

 afterwards in the insect, and, indeed, in quite another 

 manner than in the Gtistnda. 



From this it is clear that the formation of the stomachal 

 cavity in these two cases is a secondary phenomenon, 

 governed by different later conditions of the exodermal 

 and entodermal layers. The most important phenomenon 

 in both cases is the differentiation of the germ-lamellai from 

 an indifferent cell-layer, therefore that stage of development 

 represented in figs. 4 and 5. They are of great im])ortance, 

 chiefly because they represent the first processes which are 

 common to the two foiins (Ascidia and Insecta), and from 

 which the divergence of the subsequent developmental forms 

 starts. 



• The developmental states which occur iu Hydrophilus at the period 

 of the closing of the groove (see Kowalevsky, loc. cit. Taf. ix. figs. 21-25) 

 inav pcrve as an inducement for assuming the occuiTonce of the Gastrula- 

 Btage iu this animal. But to me this assumption seems to be scarcely 

 juBtitied, because the same process takes place without any such formal 

 condition in Gastropacha pirn, (See Kowalevsky, Taf. xii. figs. 1-G.) 



