334 



INVAGINATION. 



closing the hypoblast (fig. 200). This process replaces the former 

 process when the hypoblast cells are so bulky from being distended 

 by food-yolk that their invagination is mechanically impossible. 



FlG. 199. TWO STAGES IN THE DEVELOPMENT OF HOLOTHURIA TUBULOSA, 



VIEWED IN OPTICAL SECTION. (After Selenka.) 



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



;;/;-. micropyle; fl. chorion ; s.c. segmentation cavity; bl. blastoderm; ep. epiblast; 

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



There are various peculiar modifications of invagination 

 which cannot be dealt with in detail. 



Invagination in one form or other occurs in some or all the 

 members of the following groups : 



The Dicyemidae, Calci- 

 spongiae (after the amphiblastu- 

 la stage) and Silicispongiae, Cce- 

 lenterata, Turbellaria, Nemer- 

 tea, Rotifera, Mollusca, Polyzoa, 

 Brachiopoda, Chsetopoda, Dis- 

 cophora, Gephyrea, Chseto- 

 gnatha, Nematelminthes, Crus- 

 tacea, Echinodermata, and 

 Chordata. 



The gastrula of the Crus- 

 tacea is peculiar, as is also that 

 of many of the Chordata (Rep- 

 tilia, Aves, Mammalia), but 

 there is every reason to suppose 



ms 



FIG. 200. TRANSVERSE SECTION 

 THROUGH THE OVUM OF EUAXES 

 DURING AN EARLY STAGE OF DEVELOP- 

 MENT, TO SHEW THE NATURE OF 

 EPIBOLIC INVAGINATION. (After Kowa- 

 levsky. ) 



ep. epiblast ; ms. mesoblastic band ; 

 hy. hypoblast. 



