POLYEMBRYONIC DEVELOPMENT IN TATUSIA 635 



Moreover, in connection with his study on Polygnotus, Mar- 

 chal observed that the polygerm is moved back and forth in the 

 digestive tract as a result of contractions of the wall of the host. 

 He believes that this movement is analogous to the shaking of 

 Echinus eggs, and has a similar influence upon the division of 

 the germ. 



In the preliminary paper (Patterson '12) similar views were 

 given, but expressed in a somewhat different way. It was stated 

 that in all of the well known cases of polyembryony the cleavage 

 of the egg is of the 'indeterminate' type, so that it was impossible 

 to trace out a 'cell-lineage' for any particular embryo. It was 

 also stated that the primary embryo or polygerm led a sort of 

 parasitic existence, and that as a consequence it was surrounded 

 an abundance of nutritive substances. 



The cleavage of the mammalian egg is generally regarded as 

 belonging to the indeterminate type, and, although the cleavage 

 stages of the armadillo have yet to be studied, still we have no 

 reasons for believing that they will be found to differ from those 

 of other mammals; and if we may judge from the conditions of the 

 earliest stages of the blastocyst that have been examined, there 

 is no evidence to show that the early blastomeres have been 

 separated by foreign nutritive substances. The development of 

 the embryonic vesicle until the germ layers are differentiated can 

 be compared to that of certain other mammals. However, it 

 is a significant fact that at the close of the period of germ layer 

 formation the embryotrophic phase of placentation, which is par- 

 ticularly striking in the armadillo, becomes well established. It 

 may be that the nutritive substances produced by the action of 

 the embryonic nuclei upon. the maternal tissue furnish the stim- 

 ulus which excites the blastoderm to bud off the embryonic tubes, 

 just as the engorged sap of the Hessian fly is suggested by Mar- 

 chal to be the determining cause of the division of the germ of 

 Encyrtus. If this point be well taken, it is evidently not neces- 

 sary to assume that the time of stimulation to polyembryonic 

 development dates back to the early cleavage stages. 



In this*connection it might be well to call attention to another 

 suggestion that has been made. It is generally believed that in 



JOURNAL OF MORPHOLOGY, VOL. 24, NO. 4 



