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 712) 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-lmeage’ 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 
