276 



INVAGINATION. 



ficial 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 



Fig. 199. Two stages in the development oe Holothubia tubulosa, viewed 

 IN OPTICAL SECTION. (After Selenka.) 

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

 mr. micropyle ; ji. chorion ; s.c. segmentation cavity ; hi. blastoderm ; e]p. 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 

 (fig. 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. 



