82 bulletin: museum of comparative zoology. 



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 uo 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 one who chooses to make it. But several facts si^eak 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 uo such membrane can be demousti'ated in preserved material. 

 (2) In the sea-urchin and in Amphioxus, as shown by Driesch ('93) and 

 Wilson ('93), 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 

 Callidiua, investigated by Zelinka ('91), the egg is of the same form as 

 in Asplanchna, yet the cells sometimes put forth short amoeboid 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 the 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 cleavage. 



4. The inequality of the cleavage. 



5. The sequence of cleavage. (?) 



The process of which gastrulation is a part begins with the third 

 cleavage, and continues through all the period in which it is possible ta 

 trace the development cell by cell, and apparently much later. 



The [)rocess of gastrulation as above described for Asplanchna is 

 similar to the method briefly set forth by Ziegler ('95, p. 402, note) for 



