ORIGIN OF THE GERMINAL LAYERS. 



333 



into two layers, but these 

 very large number of 



two layers, in the case of a 

 ova with but little food-yolk, con- 



(fig. 198) the 

 require further 



FIG. 198. DIAGRAM 

 OF A GASTRULA. 



(From Gegenbaur.) 



a. mouth ; b. ar- 

 chenteron ; c. hypo- 

 blast ; d. epiblast. 



stitute a double-walled sack the gastrula 

 characters of which are too well known to 

 description. Following the lines of phylo- 

 genetic speculation above indicated, it may 

 be concluded that the two-layered condition 

 of the organism represents in a general way 

 the passage from the protozoon to the meta- 

 zoon condition. It is probable that we may 

 safely go further, and assert that the gastrula 

 reproduces, with more or less fidelity, a stage 

 in the evolution of the Metazoa, permanent 

 in the simpler Hydrozoa, during which the 

 organism was provided with (i) a fully deve- 

 loped digestive cavity (fig. 198 b) lined by 

 the hypoblast with digestive and assimilative 

 functions, (2) an oral opening (a), and (3) a 

 superficial epiblast (d}. These generalisa- 

 tions, which are now widely accepted, are no doubt very valuable, 

 but they leave unanswered the following important questions : 



(1) By what steps did the compound Protozoon become 

 differentiated into a Metazoon ? 



(2) Are there any grounds for thinking that there is more 

 than one line along which the Metazoa have become indepen- 

 dently evolved from the Protozoa ? 



(3) To what extent is there a complete homology between 

 the two primary germinal layers throughout the Metazoa ? 



Ontogenetically there is a great variety of processes by which 

 the passage from the segmented ovum to the two-layered or 

 diploblastic condition is arrived at. 



These processes may be grouped under the following heads : 

 1. Invagination. Under this term a considerable number 

 of closely connected processes are included. When the segmen- 

 tation results in the formation of a blastosphere, one half of the 

 blastosphere may be pushed in towards the opposite half, and a 

 gastrula be thus produced (fig. 199, A and B). This process is 

 known as embolic invagination. Another process, known as epi- 

 bolic invagination, consists in epiblast cells growing round and en- 



