454 W. B. SCOTT AND HENRY F. OSBOR^. 



analogous to that which has been pointed out in the so-called 

 embryonic rim in the Elasmobranchs.^ The cells have 

 a radiated arrangement, losing as they are reflected inwards 

 their columnar character and becoming more spindle-shaped. 

 As they approach the inner side of the lip they are quadrate, 

 then oblong, then columnar, their outer ends abutting 

 against the inner ends of the long epiblast cells. As the 

 sections pass into the lateral region of the embryo, this rela- 

 tion is lost, and confluent with the forming hypoblast cells 

 are the parent mesoblast cells. The latter may fairly be 

 considered to arise actually from the point of invagination 

 and not as a secondary splitting off from the hypoblast on 

 either side. 



Two longitudinal sections of an embryo at this period 

 have been figured in Plate XX, figs. 2 and 3. Fig. 2 represents 

 a section passing through the median line, and those changes 

 in the epiblast at the lip of the blastopore which have been 

 just referred to, may be followed. The alimentary canal has 

 not proceeded far forwards, but the cells of the upper yolk 

 are plainly forming the future hypoblast cells. The segmen- 

 tation cavity is being pressed downwards ; the section is in 

 the median line behind and out of the median line in front. 

 The reverse is true of the succeeding section (fig. 3), which 

 represents the growth of the mesoblast at the sides of the 

 invagination and the actual forward progress of the alimen- 

 tary canal in the middle line. It illustrates the position and 

 advancing obliteration of the segmentation cavity. Compar- 

 ing the two sections, a very fair idea can be formed of the 

 advance of the embryo in the early part of the stage (a). 



The process at the sides of the median line in Triton is 

 then homologous to that which Gotte" represents as occur- 

 ring in the median line in Bombinator, a construction which 

 aids him in carrying out his peculiar views of the development 

 of the notochord from the mesoblast. 



Calberla,^ on the contrary, describes as the immediate 

 result of invagination, in Ilmia temporaria, the primary 

 entoderm. This does not split in the median line, while at 

 the sides it splits soon after formation, to give rise to the 

 lateral plates of mesoderm. A fuller notice of his views 

 will be given later. 



1 Vide Balfour, ' Elasmobranch Fishes,' chap, ii, p. 43. 



'^ Vide Alexander Goette, " Eutwickelungsgeschichte der Unke," ' Atlas,' 

 Tafel. ii. 



2 E. Calberla, " Zur Entwickelung des Medullarrohres uud der Chorda- 

 dorsalis der Teliostier uud Petromyzonteu," p. 261, ' Morphologischen Jahr- 

 buch,' 3, 1877. 



