276 INVAGINATION. 



ticial epiblast (d). These generalisations, 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 differen- 

 tiated into a Metazoon? 



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

 than one line along which the Metazoa have become independently 

 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 diplo- 

 blastic 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 segmentation 

 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 



FlG. 1 ( J'J. TWO STAGES IN THE DEVELOPMENT OF HoLOTHUKIA TOBULOSA, VIEWED 



IN OPTICAL SECTION. (After Seleuka.) 



A. Stage at the close of the segmentation. B. Gastrula stage. 

 mr. micropyle ; ft. chorion; s.c. segmentation cavity; bl. blastoderm; ep. epi- 

 blast; hy. hypoblast; ms. amoeboid cells derived from hypoblast; a.e. archenteron. 



invagination. Another process, known as epibolic invagination, 

 consists in epiblast cells growing round and enclosing the hypoblast 

 (tig. 200). This process replaces the former process when the hypo- 

 blast cells are so bulky from being distended by food-yolk that their 

 invagination is mechanically impossible. 



There are various peculiar modifications of invagination which 

 cannot be dealt with in detail. 



