Dr. W. Salensky on Héickel’s Gastrea Theory. 5 
Of the embryonic development of the Turbellaria also we 
know very little; and what we do know does not prove that 
these animals pass through a Gastrula-stage. As far as I know, 
there exist only two investigations which show thoroughly and 
in detail the embryology and especially the production 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 ft. 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 with 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-stage, because in the sexually immature 
state they differ very little in their organization from the G'as- 
trula type. 
It is otherwise with Nemertina, in which, by the remarkable 
investigations of Mecznikofft, 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 egg. This vesicle then under- 
goes an introversion, which subsequently becomes differentiated 
into two parts, the anterior intestine and the stomach. Here, 
therefore, we have a G'astrula-stage. The Nemertina, how- 
ever, must be separated from the other Turbellaria, as they must 
be referred to the Ccelomati, and the others to the Accelomi. 
Whether a Gastrula form exists in the ontogeny of the Ne- 
matoda is not yet proved. From the researches of Leuckart § 
* Beitrage zur Anatomie und Entwickelungsgeschichte einiger See- 
planarien von St. Malo, 1868. 
+ “ Embryogénie des Planaires d’eau douce,” Archives Néerlandaises des 
Sci. &c. This memoir is known to me only by the reports of Keferstein 
and Leuckart. 
t Mém. de l’Acad. Imp. de St. Pétersb. tome xiii. 
§ Leuckart, ‘ Die menschlichen Parasiten,’ Bd. ii. Lief. 1, p. 93. E. van 
Beneden (Recherches, &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, 
