XIII 



EOTIFEEA 



425 



dorsal side of the foot, a hollow invagination of ectoderm appears, 

 this is the proctodaeum, that is the rudiment of the anus and 

 hind-gut. 



■We have already seen that the hinder endodermal rudiment is 

 contained in the foot, but before the anus is formed it begins to be 

 displaced by a solid terminal invagination of ectoderm, which is the 

 rudiment of the foot gland. 



In the following period of development the head region under- 

 goes complicated developments. The proboscis is bent ventrally. 

 Between it and the cuticular strips, mentioned above, a solid invagina- 



eAd"" 



FiQ. 336. — Optical sagittal sections through two late embryos of Callidina russeola iu 

 order to show the formation of organs. (After Zelinka. ) 



A, stage of predominant growth of the tail, B, later stage : predominant growth of the head 

 region, eaud.gl, foot gland ; cer^, cer^j first and second rudiments of the cerehral ganglion ; endX, endo- 

 derraic rudiment of the gut ; end^, endodermic rudiment of the excretory bladder ; gon, genital rudi- 

 ment ; prob, rudiment of proboscis : proct, proctodaeum ; sal, salivary gland ; siomi, inner stomodaeum 

 or mastax ; sttm?, outer stomodaeum ; ten, tentacle ; tr, border of region of apical plate which gives 

 rise to the " wheels " ; troph, rudiment of one of the trophi or jaws. 



tion of ectoderm appears on each side of the trochal disc. Then the 

 whole head region is retracted and surrounded by an upgrowth of 

 the ventral lip, which, however, does not quite reach the dorsal side. 

 Later this upgrowth develops the outer cilia known as the 

 cingulum. Still later, however, the whole head region — proboscis, 

 and apical shelf, is again protruded ; and the cuticular strips are 

 specially thickened at each side, where the shelf shows an indentation. 

 The ectodermal thickenings between these and the proboscis are 

 protruded as hemispherical bosses. Subsequently these are again 

 invaginated, and remain so till the embryo is born, when they are 

 finally protruded. They are then surrounded with cilia, developed 



