ox THE FORMATION OF THE GERIMIXAL I,AYEKS IN PETROMYZON. 



131 



I. The Gastrulation. 



My <)l).sei'vah<)iis on ^^eginetitiiig' ("-gg^i are too iiicoinj)]('r(' to 

 eiialjle me to make any definite ytuiement in regard to the process. 

 The earliest staire on which I wish to ofier remarks is that of the 

 oldest morula. \\ hen this is viewed as an o{)a<jue oljjeei", the animal 

 pole and the vegetaHve pole ean ])e exiernally distinguished: tlie former 

 is whitish and somewhat translucent hoth in fresli and in hardened 

 specimens, w^hile the latter is opa(p!e and of a p>ale yellow colour. A 

 slio-ht depression o'oes round the eo-o- niarkini»- the Ijoundarv between 

 these two portions. 



In sections (tig. 1) the contrast of the two portions is well 

 l)rought out. A spacious segmentation cavity is placed excentrically 

 in tlie egg and occupies the whole interior of the anim;d pole. The 

 cells forming the roof of this cavity are arranged in 2-4 layers and, 

 compared with the cells of the opposite pole, are smaller, comparatively 

 free from yolk gj-anules^-and have smaller nuclei VN'hich stain ori the 

 whole deeper. The cells forming tlie floor of tlie segmentatioii cavity 

 are 7-8 deep, full of yolk-granules with other cliaracters as indicated 

 above. All the cells are po!yg<vnal in sliape, evidently througli mutual 

 pressure. 



The next step of development consists in the gradual thinning 

 out of the roof of the segmentation cavity. ()])inions differ iri regard 

 to the extent to Avhich this process is carried before gastrulation sets 

 in. According to Scott^ and Shipley," the roof of tlie cavity becomes 

 one cell layered before the gastrulation begins. On the other hand. 



1. W. B. ScoU : -Boitr;i;4e zur Entwicklungsgeschichte der Petrouiyzon. Morph. Jahr)). 

 Bd. VII. 



2. A. E. Shipley:— On sotii'^ points iti the Developuient of Pi'li-om'izon tlui:i<itili>i. gii:i''"fc- 

 Jouru. Micr. Sei., Vol. 27. 



