Dr. W. Salensky on IldckeVs (jriistrtea Theory. 5 



Of tlie embryonic development of the Turbellaria also we 

 know very little ; and what we do know does not prove that 

 these animals pa.ss throut^h a Gastruia-Htai^c. As far as 1 know, 

 there exist only two investigations which show thoroughly and 

 in detail the embryology and especially the ])roduction of the 

 organs, of the Turbellaria. In the memoir of E. van Beneden 

 (Recherches &c.) the process of segmentation is chiefly con- 

 sidered. The two other investigations are due to Keferstein* 

 and Knappert f. According to the last author the vitelline 

 mass undergoes segmentation and then separates into a cen- 

 tral and a peripheral layer, of which the latter, by repeated 

 division, furnishes an animal lamella, which becomes converted 

 into the body-wall witli the muscular layer and epithelia, and a 

 vegetative lamella, which is developed into the intestinal mem- 

 brane. There is little in this memoir upon the production of 

 the buccal orifice and intestinal cavity. Keferstein's investi- 

 gations agree pretty nearly with those of Knappert, as he also 

 represents the body-wall and the intestinal wall as produced by 

 the division of a layer, the upper layer. It seems to me, how- 

 ever, that in the Turbellaria we may with great certainty as- 

 sume the Gastrula-stRge, because in the sexually immature 

 state they differ veiy little in their organization from the Gas- 

 tfula type. 



It is otherwise with Nemertina, in which, by the remarkable 

 investigations of Mecznikoff J, the earliest developmental pro- 

 cesses have been elucidated. From these interesting researches 

 we learn the important fact that the larva is excluded in the 

 form of a vesicle of one layer and that it leads a free life. 

 According to Mecznikoff a vesicle of one layer is first produced 

 from the egg of the Nemertian ; this becomes covered with 

 cilia and then escapes from the t'gg. This vesicle then under- 

 goes an introversion, which subsequently becomes differentiated 

 into two parts, the anterior intestine aud the stomach. Here, 

 therefore, we have a Gastrula-s,ta.ge. The Nemertina, how- 

 ever, must be separated from the other Turbellaria, as they must 

 be referred to the Ccelomati, and the others to the xVcoelomi. 



Whether a Gastrula form exists in the ontogeny of the Xe- 

 matoda is not yet proved. From the researches of Leuckart § 



• BeitrJige ziir Anatomie unci Entwickelungsgeschiclite emiger See- 

 planarien von St. Malo, 1^08. 



t " Embryogenie des Planaires d'eau douce," Archives Neerlandaises des 

 Sci. &c. This memoir is known to me only by the reports of Keferstein 

 and Leuckart. 



X M^m. de I'Acad. Imp. de St. Petersb. tome xiii. 



§ Leuckart, * Die menschlichen Parasiten,' Bd. ii. Lief. 1, p. 93. E. van 

 lieneden (Reclierche?, &c. p. 102) regards the interior opaque mass of the 

 embryo as nutritive vitellus ; but this appears to me to be by no means 

 proved. 



