180 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



The tii si rudiment of the organ is seen to be a simple pocket-like 

 outgrowth of the gut -wall (A, c.o) : this becomes more and more 

 prominent (B, C) : it becomes gradually constricted off at its base 

 from the gut-wall, its cavity becoming isolated first (D). Finally it 

 separates completely from the main endoderm and its outer end 

 undergoes fusion with the deep layer of the ectoderm. Its cavity 

 then opens to the exterior and the fully functional condition is 

 reached the endodermal origin of the secretory lining being for a 

 time betrayed by the conspicuous persistent yolk granules in its cells. 



It will be noted that the exposed side of the secretory epithelium, 

 that on which the secretion is extruded, is that which originally 

 faced inwards towards the lumen of the alimentary canal. In other 

 words the direction in which the extrusion takes place is morpho- 

 logically the same as that of any other part of the glandular lining 

 of the gut- wall. 



As is the case in other forms the cement-organ is a transient, 

 purely larval, structure. About stage 31 (Fig. 197, D) degeneration 

 commences : the gland shrivels up, the gland-cells becoming more 

 slender and dark pigment making its appearance in their interior, 

 the epithelium becomes penetrated by ingrowing blood-vessels, its 

 cell-boundaries become indistinct. The process of atrophy goes on 

 rapidly and by stage 36 (Fig. 197, F) the organ has completely 

 disappeared. 



An interesting variation from the normal course of development 

 is found in specimens in which the cement -organ rudiments are 

 more or less approximated to one another. This variation reaches 

 its maximum in occasional individuals in which they are completely 

 fused and form an unpaired structure, continuous across the mesial 

 plane. 



In the actinopterygian Ganoids the cement-organ develops along 

 the same general lines as those just indicated. In the Sturgeons 

 the development has been worked out recently by Sawadsky (1911) 

 in Acipenser ruthenus. Here the organ forms a rounded projection, 

 very much in the same position as that of Polypterus, but in this 

 case each becomes divided by a groove so as to form two rounded 

 knobs. These knobs eventually grow out to form the tactile barbels 

 of the adult, the secretory epithelium being carried out on the 

 surface of the barbel as it grows. 



The secretory epithelium is here also endodermal, its rudiment 

 being the gut- Wall immediately dorsal to the position in which the 

 mouth will develop later and bein^ continuous across the mesial 

 plane. The unpaired condition which occurs in /'i>/i/-/iterus as a 

 variation is thus normal in the ease f the sturgeon. As the head 

 increases in length the BeCFOtOIJ epithelium becomes carried out on 

 . ntral surface, looking just as if it were the thickened ectoderm 



of this surface. Finally the paired condition comes about, the lateral 



part- ol' the BecretOTy epithelium coining to be supported by the 

 knob like projections already mentioned. 



