THE GASTR^A THEORY AND ITS SUCCESSORS. 



85 



formation of the diblastula must be regarded as having 

 been primarily adaptive, but later became dependent 

 upon heredity. The physiological molecules composing 

 the ovum tended to become differentiated earlier and 

 earlier in the ontogenetic history, those destined to form 

 the entoderm being early set apart for that purpose. This 

 is Lankester's doctrine of precocious segregation, equiv- 

 alent to the law laid down by Haeckel as "heterochrony 

 in the palingenetic phenomena of ontogeny." Lankester 

 supposes that in invaginate types the segregation of the 

 endoderm extends to the first division of the ovum, one 

 spherule containing the ectodermal molecule and the 

 other the entodermal (Fig. i). By the continued division 

 of these spherules a num- 

 ber of cells are produced, 

 the entodermal ones ar- 

 ranging themselves with- 

 in the ectodermal, a gas- 

 trula being thus formed 

 (Figs. 2, 3) without the 

 intervention of a blastula. 

 This structure, which oc- 

 curs so frequently in the 

 typical formation of an 

 invaginate gastrula, Lankester considers to be an alto- 

 gether different structure from that which precedes the 

 diblastula, and to have been secondarily acquired by 

 the mechanical accumulation of fluid between the cells 

 of the forming gastrula, whereby the endoderm cells are 

 forced out from their position within the ectoderm, and 

 a cavity is thus formed between the two layers (Fig. 4). 

 This cavity is, however, not equivalent to that of the 



Figs. 1-4. 



