32 J. GRAHAM KERR. 



ectoderm in Amphioxus does to the method occurring more 

 usually by the formation of a neural groove. The method of 

 chorda formation found in Lepido siren may be compared, 

 on the other hand, with the modification of the development 

 of the neural rudiment occurring in Teleosts. 



Growth of Epiblast. — In that the epiblast grows at its 

 edge by delamination, Lepido sir en agrees with what has 

 been found in various Amphibia (Houssay, Robinson and 

 Assheton, Gronroos), but differs from what has been found 

 by Brauer in Gymnophiona, 



Mesoblast Formation. — In regard to the development 

 of mesoblast there are two features of special interest. The 

 first of these is the fact which cannot, I think, be doubted 

 by anyone in Lepidosiren — that the so-called " gastral " 

 mesoderm is formed directly out of the smaller blastomeres 

 on each side of the archenteron. These masses are con- 

 nected aci'oss the middle line, and the common rudiment of 

 mesoderm and chorda is quite continuous. To go further 

 than this and say, as has been done by others, that the 

 notochord is derived from mesoderm, is quite unwarranted. 



I do not see any possible phylogenetic explanation of this 

 phase in the formation of the mesoderm. 



The later phase in its development which is of interest is 

 that in which we see the mesoderm as a sheet upon each side, 

 segmented or not according to its age, free at its inner 

 thicker edge next the chorda, and thinning away to become 

 continuous with the large yolk-cells or primitive hypoblast at 

 its outer edge, where it continues to grow by delamination. 

 Here we have a condition of things upon which I think a ray 

 of light is thrown if we regard it as a fleeting reminiscence 

 of the primitive method of mesoderm development in the 

 Chordata. 



The phenomena, in fact, in Lepidosiren, closely paralleled 

 by those in Petromyzon, suggest a scheme of the steps by 

 which the method of mesoderm formation in the higher ver- 

 tebrates may have been derived from that found in Am- 

 phioxus, differing somewhat from that due to Hertwig. 



