THE RECAPITULATION THEORY 323 



One of the most recent attempts to explain 

 developmental peculiarities as due to mechanical 

 causes is Mr. Dendy's suggestion with regard to 

 the pseudogastrula stage in the development of 

 calcareous sponges. It is well known that while 

 the larva is in the amphiblastula stage, and still 

 imbedded in the tissues of the parent, the granular 

 cells become invaginated within the ciliated cells, 

 giving rise to the pseudogastrula stage. At a 

 slightly later stage, when the larva becomes free, 

 the invaginated granular cells become again everted, 

 and the larva spherical in shape ; while still later 

 invagination occurs once more, the ciliated cells 

 being this time invaginated within the granular 

 cells. The significance of the pseudogastrula stage 

 has hitherto been undetermined, but Mr. Dendy 

 points out that the larva always occupies a definite 

 position with reference to the parental tissues ; that 

 the ciliated half of the larva is covered by a soft 

 and yielding wall, while the opposite half, composed 

 of the granular cells, is covered by a layer stiffened 

 with rigid spicules ; and his observations on the 

 growth of the larva lead him to think that the 

 pseudogastrula stage is brought about mechanically 

 by flattening of the granular cells through pressure 

 against this rigid wall of spicules. 



Embryology supplies us with many unsolved 

 problemgjmcHt is not to be wondered at that this 

 should be the case. Some of these may fairly be 

 spoken of as mere curiosities of development, while 

 others are clearly of greater moment. I do not 

 propose to catalogue these, but will merely mention 



