S6 MARINE BIOLOGICAL LABORATORY. 



delaminating blastula, inasmuch as it does not become 

 the archenteron on the formation of the endoderm, but 

 is obUterated by the invagination. It is necessary then 

 to distinguish between the delaminating blastula and 

 the invaginating " pseudo-blastula," and between the 

 archenteric blastocoel of the former and the *'pseudo- 

 blastocoel " of the latter. 



The ingenuity of this theory is its strong point, but 

 simplicity can hardly be considered one of its charac- 

 teristics. It has not met with the general acceptance 

 which greeted its predecessor and rival, nor has it had 

 the same influence on embryological investigation, — a 

 result owing to the fact that no evidence in support of 

 such an origin of the gastrula can be found. 



A third theory is due to Balfour, and may be termed 

 the AmpJiiblastula theory.^ It is founded upon the 

 peculiar blastula of the calcareous sponge Sycandra, the 

 cells of which at one pole are columnar and ciliated, 

 while those of the other pole are larger and granular. 

 Balfour thinks it possible to consider this larva as a 

 colony of Protozoa one-half of the individuals of which 

 have been specialized for locomotor and respiratory pur- 

 poses, while the others are essentially nutritive. In the 

 later stages, however, the ciliated cells become invagi- 

 nated within the granular ones, a fact which seems at 

 variance with the theory if the granular cells are to be 

 homologized with the endoderm of other forms, but which 

 Balfour explains in the following manner. On the set- 

 tling down and fixation of the sponge embryo the ciliated 

 cells, being partly locomotor in function, become, to a 



1 F. M. Balfour. A Treatise on Comparative Embryology, vol. i. Lon- 

 don. 1880. 



