II 



THE SKIN AND ITS DEEIVATIVES 



79 



The flask-glands of Lung-fishes and Amphibians develop in the 

 first instance as solid local proliferations of the deep layers of the 

 epidermis which grow down into the subjacent connective tissue of 

 the dermis and form a lumen by secondary excavation. The fully 

 developed flask-gland is ensheathed in a coat of smooth muscle-fibres 

 and it is an interesting fact that these are believed to be developed 

 from the ectodermal cells of the gland-rudiment. 



Cement-organs of apparently ectodermal origin occur in two out 

 of the three surviving lung-fishes — Protopterus and Lepidosiren — 

 and form conspicuous features during late embryonic and larval life 

 (see Fig. 200, F, Chap. VII.). 



In an embryo of Lepidosiren three days before hatching the 

 cement-organ forms a crescentic structure stretching across the mid- 



Fig. 48. — Embryos and larvae of Bufo vulgaris to show the cement-organ upon the 

 ventral surface. (After Thiele, 1887.) 



ventral line with its concavity forwards, just behind the position in 

 which the mouth will appear later. About stages 32-34 the organ 

 is at its maximum development and forms a large prominently pro- 

 jecting structure ventrally below the opercular region. Towards the 

 end of larval life the cement-organ commences to atrophy, the process 

 being furthered by its invasion by crowds of phagocytes, and in a 

 short time the organ has completely disappeared. 



The cement-organ is a derivative of the deep layer of the epi- 

 dermis. It commences as a slight thickening of this layer (Fig. 

 47, A) the cells assuming a tall columnar form. These columnar 

 cells become the secretory cells while the superficial layer of the 

 ectoderm breaks down over them so as to expose their outer ends 

 (Fig. 47, B and C). It is to be noted that there is no trace whatever 

 of any connexion of this cement-organ with the endoderm : it is 

 ontogenetically a purely ectodermal structure (see, however, p. 181). 



Cement-organs of very similar appearance are found in the larvae 



