Dr. W. Salensky on IldckeVs Gastraja Theory. 7 



ones soon make their appearance. Tlic former afterwartU 

 form the entoderm, the hitter the dermal lamella. Between 

 these two foundations of the germ-lamellie a third layer of cells 

 is immediately produced, and this forms the middle g(!rm- 

 lamella. From this it follows that in Euaxes there is no stage 

 which has any resemblance to the Gastrceay and, indeed, that no 

 such stage can occur, because the Gastrula should consist only 

 of two layers. But here, even before the conclusion of the 



?rocess of segmentation, all three germ-lamellie are founded, 

 n Lumhricusj however, which is systematically very nearly 

 allied to Euaxes^ a Gastrula-stnge occurs. 



In the ontogeny of the Chifitopoda there seems to be no 

 GastruIa-stAgc. From the investigations of Clapar^de and 

 Mecznikoff* it appears that, after the segmentation of the egg 

 in the Chsetopoda, an embryo consisting of two germ-lamellae 

 is formed, and that this soon acquires the bands^or tufts of cilia 

 &c. and then quits the egg without possessing any stomachal 

 cavity. Then the eyes are formed in the embryo, the seg- 

 ments of the body become differentiated, and finally the sto- 

 machal cavity with mouth and anus is jnoduced. The latter 

 consequently takes place long after the time when the embryo 

 already possesses its larval organs {Sjjio fuligiHOSus, Lumhri- 

 conereis sp., and Dasyclione lucullana). 



I need hardly mention that the Ascidia pass through a Gas- 

 tri(Ia-sta.ge in their ontogeny. This is fully proved by the well- 

 known researches of Ko%valevsky. 



According to this examination of the stock Vermes, we can 

 with certainty detect the Gastrula-stage only in Sajitta, Pho- 

 romsy Lumbricus, the Ascidia, and the Nemertina. In the 

 other Vermes the existence of the Gastrula-sta.ge is by no 

 means proved by embryological researches. We have seen 

 that it does not occur in the ontogeny of most Trematoda, 

 and probably of most Nematoda, Bryozoa, Euaxes, and the 

 Chffitopoda. Perhaps the Leeches might be added to the 

 Vermes w^hich pass through a Gastrula-stage (Leuckart, ' Die 

 menschlichen Parasiten,' Bd. i. p. 689). 



Before we pass to the other types of animals, we must refer 

 to an important phenomenon which is of much significance in 

 the correct estimation of the factual evidence. The mode of 

 formation of the buccal aperture and of the anterior part of 

 the intestinal canal must be mentioned, as in it phenomena 

 occiu: which might lead to the assumption of a Gastrula- 

 stage in cases in which really no such stage exists. In the 

 ontogeny of all; animals (except the Sponges and some 



* " Beitriige zur Erkenntniss der Entwickelungrsgeschichte der Ohae- 

 topoden," Zeitschr. fiir wiss. Zool. Bd. xix. pp. 109, 182, and lU?. 



