GERMINAL LAYERS OF THE EMBRYO. 
249 
provided with (1) a fully developed digestive cavity (fig. 1 h) 
lined by the secretory hypoblast, (2) an oral opening (a) , and 
(3) a superficial 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 
differentiated into a Metazoon ? 
(2) Are there any grounds for thinking that there is more 
than one line along which the Metazoa have become inde- 
pendently differentiated from the Protozoa? 
(3.) To what extent is there a complete homology 
between the two primary germinal layers throughout the 
Metazoa ? 
Ontologenetically there are 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 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. 2, A and 
b). This process is known as embolic invagination. Another 
Fig. 2. — Two Stages in the Development of Holothuria tubulosa, viewed in 
optical section. (After Selenka.) A. Stage at the close of the seg- 
mentation. B. Gastrula stage, mr. micropyle ; chorion; 5. 6*. seg- 
mentation cavity ; hi. blastoderm ; ep. epiblast ; hy. hypoblast ; ms. 
amoeboid cells derived from hypoblast ; a.e. archenteron. 
process, known as epibolic invagination, consists in epiblast 
cells growing round and enclosing the hypoblast (fig. 3). This 
