82 BULLETIN: MUSEUM OF COMPARATIVE ZOÜLOGY. 
seems to me inexplicable on any simple mechanical ground ; the form at 
this stage apparently must be judged from the same standpoint as that 
of the adult, which no one would attempt to refer to a simple mechanical 
principle. Of course the assumption of the presence of a non-elastic 
non-extensible membrane of ellipsoidal form, which later becomes elastic 
and extensible, would explain the retention of the shape, and is open to 
any oue who chooses to make it. But several facts speak against this 
view, aside from the general improbability of the existence of a membrane 
of such peculiar and changing qualities : —(1) The negative evidence 
that no such membrane can be demonstrated in preserved material. 
(2) In the sea-urchin and in Amphioxus, as shown by Driesch (93) and 
Wilson (793), development takes place as well when the membrane is 
removed as when present. This of course does not show that the same 
is true for Asplanchna, but it does show that the importance of the egg 
membrane has been overestimated for some cases. (3) In the rotifer 
Callidina, investigated by Zelinka (91), the egg is of the same form as 
in Asplanchna, yet the cells sometimes put forth short amœboid pro- 
cesses, which of course would be impossible with a close membrane. 
(4) In another rotifer, Melicerta ringens, the egg is not a regular oval or 
ellipsoid of rotation, but one side is flattened while the other is curved, 
and this irregular form is retained during the shifting of the blastomeres, 
as is the case in Asplanchna (see tho figures of Zelinka, '91, and of Joliet, 
83). Such a form would not be preserved even by such a non-elastic 
membrane. 
The facts given under (3) and (4) seem to me to render entirely in- 
admissible the explanation that the form of the egg in Asplanchna is 
due to the presence of a membrane,.since this would leave the exactly 
similar phenomenon in the related forms Callidina and Melicerta without 
explanation. 
The factors concerned in gastrulation may be summarized as follows : — 
1. The form of the egg. 
2. The change in the form of the cells at cleavage. 
3. The direction of cleavago. 
4. The inequality of the cleavage. 
5. The sequence of cleavage. (7) 
The process of which gastrulation is a part begins with the third 
cleavage, and continues through all the period in which it is possible to 
trace the development cell by cell, and apparently much later. 
The process of gastrulation as above described for Asplanchna is 
similar to the method briefly set forth by Ziegler (95, p. 402, note) for 
