CHAPTER VI. 



The changes from Stage G to Bikth. 



The changes which take place during and subsequent to 

 Stage G are mainly changes of growth and histological differ- 

 entiation. The most important organs which have not yet 

 made their appearance by the close of Stage f, are the crural 

 glands and tracheae. The origin of the latter is, I regret to 

 say, still hidden from me. The remaining organs have acquired, 

 in all essential respects, the adult relations by the close of 

 Stage F. 



*D"- 



The Ectoderm. 



In Stage g, the ectoderm retains the characters already 

 described in Ch. Ill, p. 54. It forms au extremely thin, 

 much vacuolated layer over the greater part of the body, and 

 the nuclei are far apart and in a single layer (PI. X, fig. 5). In 

 embryos of this age, the ectoderm does not contract when the 

 embryo is preserved, and no doubt its extreme tenuity is due 

 to this fact. On the ventral organs the ectoderm is thicker 

 and the nuclei in more than one layer and close together. 

 On the dorsal hump also, the ectoderm still remains thick, 

 with a large amount of protoplasm external to the nuclei. 

 The dorsal hump, however, has already begun to atrophy ; it 

 eventually completely vanishes. 



The general ectoderm possesses a large number of vacuoles, 

 and in certain places the nuclei are aggregated together in 

 masses, and are smaller than elsewhere, forming the rudiments 

 of the future spiniferous sense-organs. The latter give rise to 

 the white spots seen on the skin of embryos of this age. 



In P. Balfouri, and, to a slight extent, in P. capensis, 



K 



