380 Dr. G. Biitschli on the Gastrea-Theory. 
the delamination-gastrula. In this case, just as we see in 
the Volvox-blastula at the present day *, a contraction of the 
still unilamellar cell-plate occurred betore the formation of 
the entoderm, and the formation of the entoderm took place 
only subsequently within the closed blastula. The advan- 
tages of such an anticipatory sphere-formation would be the 
same that have been already urged in regard to the develop- 
ment of the invagination-blastula. But, should this concep- 
tion of the delamination-gastrula prove to be correct, an 
essential difference from the invagination-blastula must have 
shown itself in the history of its production, or been able to 
show itself, as it is conceivable that this difference may be 
cancelled by secondary variations. While the invagination- 
blastula is formed by the separation or divergence of the 
segmentation-cells (Pl. XV. figs. 2a, 3a, 3b), the delami- 
nation-blastula must, on the contrary, have originated by a 
process of incurvation like that which leads to the formation 
of the blastula of Volvow (Pl. XV. figs. la-1d). The mate- 
rials at my command upon the history of the formation of the 
delamination-blastula furnish no certain data for the settlement 
of this question, but at the same time they are not adverse to 
our conception. From the representation that Fol T gives of 
the production of the delamination-gastrula of the Geryonida, 
our conception may perhaps obtain some support; at least, in 
his fig. 5 Fol represents a 16-celled stage, which closely agrees 
with the corresponding stage of Volvox. Although the figure 
does not show this quite certainly, it nevertheless seems to be 
pretty well indicated that the four cells of the inferior sur- 
face do not close together, and therefore the already existent 
blastula-cavity still communicates through an inferior aperture 
with the outer world, just as in Volvox. In general, however, 
little attention has hitherto been paid to this not unimportant 
question in the formation of the blastula. 
Asa matter of detail we have still the question, which state 
of the delamination-gastrula are we to regard as the more 
primitive, that with an original primitive intestinal cavity or 
that with a solid entodermal mass, the so-called “ parenchy- 
mula” of Metschnikoff? In accordance with the hypothesis 
to which we have hitherto adhered, we must, with Lankester, 
* On the development cf this see Goroshankin, “Genesis im Typus der 
palmellenartigen Algen,” in Mitth. d. k. k. russ. Ges, naturf. Freunde zu 
Moskau, Bd. xvi. (in Russian ),and Kirchner, “ZurEntwicklungsgeschichte 
von Volvox minor” in Cohn’s Beitr. zur Biologie der Pflanzen, Bd. iii. 
A connected exposition of the reproduction and developmental phenomena 
of the Volvocinez and their allies will be found in the next section, on 
the Flagellata, in my Protozoa. 
+ Jenaische Zeitschrift, Bd. vii. p. 471, Taf. xxiv. 
