452 J. GRAHAM KERR. 



must recognise in the transient phase which I have described 

 the indication of a far back phylogenetic stage in the history 

 of vertebrate muscle. 



The facts which I have summarised above in regard to the 

 development of nerve and muscle in Lepidosiren support, 

 on the whole, the theoretical views of Hensen in regard to 

 the development of nerve, and are totally opposed to the 

 views of Bidder and Kupffer and of His that the nerves 

 PTOw out towards their motor end structures, and unite with 

 them secondarily. They also lend additional support to the 

 view which Sedgwick has consistently taught, and which has 

 been supported by much recent work, such as above all that 

 of Apathy upon the structure of the nervous system, that we 

 must regard the vertebrate individual in the first place as an 

 organic and continuous whole, whose various regions are 

 linked up in organic continuity, rather than as an aggregation 

 of separate cells or organs. 



SUMMAEY OF THE MORE iMrORTANT FaCTS DESCRIBED. 



1. Certain of the ectoderm cells are provided with tail-like 

 processes which pass into a subepidermal layer, into which 

 also pass processes of the underlying mesenchyme cells. 



2. The flask-shaped glands of the skin arise as solid thick- 

 enings of the deep layer of the epidermis which develop a 

 lumen later. 



3. The cement organ arises as a thickening of the deep layer 

 of the ectoderm over which the superficial layer degenerates 

 and disappears. Its atrophy is caused mainly by the action 

 of phagocytes. 



4. The chromatophores of the skin are mesodermal in 

 origin. 



5. There is no invagination of ectoderm to form a true 

 stomoda3um. The ejiithclium of the buccal cavity is developed 

 in situ from the outer layer of the solid anterior portion of 

 the yolk-laden enteric rudiment. 



