DEVELOPMENT 



219 



Owing to the elongation of the body within the narrow 

 space of the egg the hinder part is bent up on the ventral 

 surface (D, E) ; and this part, narrower than the rest, forms the 

 foot, the centre of which is at first occupied by a column of 

 hypoblast. The cloaca is now formed by a dorsal ingrowth 

 of epiblast (the " proctodaeum ") at the junction of the foot 

 and the body (nil). The hypoblast in the body anterior to the 

 cloacal ingrowth forms the digestive apparatus ; the part imme- 

 diately behind forms the reproductive organs (o) ; and the hind- 

 most part apparently disappears. An ingrowth of epiblast at 

 the extreme tip of the foot gives rise to the cement glands (/#). 



n TIG. 114. Development of 



Callidina. (After 

 Zelinka.) A, Early 

 stage showing involution 

 of granular cells (</), 

 to form the mastax or 

 gizzard. B, Involution 

 complete. C, Second 

 involution of epiblast 

 cells to form pharynx. 



D, The embryo bent on 

 itself at ventral fold (rf). 



E, Showing ingrowth of 

 epiblast to form brain 

 (br) : an, involution of 

 epiblast to form cloaca ; 

 br, brain ; ep, epiblast ; 

 fg, involution to form 

 cement glands of foot ; 

 y, granular cells ; gi, 

 gizzard ; hyp, hypoblast ; 

 in, mouth ; o, ovary ; 

 sp, salivary glands ; if, 

 limiting body from foot. 



The muscles arise from the epiblast cells. The disc arises from 

 the modification of epiblast cells lateral to and behind the 

 mouth, enclosing a so-called " polar area " ; it is completed by 

 the transformation of cells on the ventral side of the mouth. 

 The brain (Z>r) is formed by the multiplication of epiblast cells ; 

 and in Bdelloida a ventral ingrowth below the mouth forms the 

 sub-oesophageal ganglion. The ciliated cup in Mdicerta is 

 formed as a ventral hollow, only later on united with the ciliated 

 furrow of the wreath by the lateral grooves. 1 In Mdicerta the 

 two eyes are formed in the polar area. The young as hatched 



1 It does not appear to us that Zelinka is justified by his account of the 

 development in regarding this cup as other than a part of the disc. 



