No. I.] THE EMBRYOLOGY OF THE UNI ONI DAE. 41 



cavity of Pilidium from the ectoblast. I must call attention to 

 Goette's figures of Anodonta, in which scattered mesenchyme 

 cells are shown in the place occupied by larval mesoblast in 

 Unio, and which could hardly have come from the teloblasts of 

 the mesoderm. Compare, also, the position of the dissociated 

 mesenchyme cells in front of the archenteron in Cyclas with 

 the position of the larval mesoblast in Unio (text, Fig. 7). 

 Fol held that the ectoderm contributed to the mesoblast in 

 pteropods, heteropods, and pulmonates. His observations may 

 still be partly true even though Knipowitsch (No. 49) has seen 

 mesoblast pole-cells in Clione, and Rabl in Planorbis. May 

 there not, too, be some glint of truth in Sarasin's wholesale 

 deduction of mesoblast from ectoderm in Bithymia tentaculata, 

 in spite of the fact that Erlanger (No. 43) has seen the pole- 

 cells of the mesoblast, and has traced them back to the poste- 

 rior macromere } Wilson has traced back the head-kidneys of 

 Nereis to two ectoblastic cells ; Kleinenberg (No. 48) believed in 

 ectomesoblast for Lopadorhynchus. In fact, it would be weari- 

 some to review all the statements in support of the ectodermal 

 origin of some mesenchymal cells, which one could cull from 

 the literature. Unfortunately, most of the statements are 

 qualified {cf., e.g., Ziegler's remark, supra), and doubt has been 

 thrown on the rest. It may be, however, that the pendulum 

 has now swung too far in the other direction. I believe 

 in the complicity of the ectoderm in the formation of the 

 mesenchyme. The coelenterate ancestors of the Mollusca 

 possessed mesectoderm cells of contractile function. It would 

 not be very strange if the undoubted phyletic continuity of 

 ectoderm and mesoderm should repeat itself in ontogeny. 

 (Cf. Kleinenberg, No. 48, p. 202, etc^j 



As to the question of the relation of the embryonic axes to 

 the first and second cleavage planes, it seems to me that too 

 much emphasis has been laid on one point, viz. : on the rela- 

 tion which the entomeres bear to the embryonic axes. There 

 is a certain justification for this, inasmuch as the entomeres 

 are as a rule so much larger than the ectomeres. But the fact 

 that the ectomeres are given off in different directions from the 

 entomeres has not entered into account apparently. In Crepid- 



