( 802 ) 



to gastmlation proper as we are to do that of the covering of the 

 yolk by the entoderm in sauropsids. Daring this longitudinal growth 

 of the embryo new cells are produced by the prostomal thickening 

 and pushed inwards to form the entoderm, but this may not be 

 called gastrulation any more. The period of development, during which 

 the yolk is being covered by the blastodermring, differs much in 

 different embryos. In muraenoids at the time the yolk-blastopore is 

 closed the embryo possesses from 5 to 10 pairs of primitive segments; 

 the issuing larvae possess 58 to 75 segments. In salmonidae at the 

 closure of the yolk-blastopore of the 57 to 60 segments 18 to 28 

 are differentiated. The other organs too are developed to a greater 

 or lesser degree. To use the term gastrulation for the processes 

 during this whole period of development leads us into difficulties. 



THe first question we have to answer, when we study closer the 

 process of gastrulation in teleosts, is: at what time does the process 

 of gastrulation begin in the large meroblastic eggs? 



Recently Brachet l ) has called attention to a process, which he 

 calls "clivage gastruléen", and which he describes for the eggs of 

 Rana fusca as the formation of a circular groove at the base of the 

 segmentation-cavity around the yolk-mass, before there is to be seen 

 a trace of a blastopore (Rusconic groove) at the outside of the egg: 

 "immédiatement') avant que la gastrulation ne commence, la eavité 

 de segmentation, sphérique on a pen prés, occupe l'hémisphère 

 supérieur de l'oeuf (de Rana fusca).... Bientót, sur tout Ie pourtour 

 du plancher de la eavité de segmentation, une fen te se produit par 

 clivage ; cette fente 's enfbnee entre les cellules de la zone marginale 

 et les divise en deux couches : l'une, superficielle, prolonge directement 

 la voute de la eavité de segmentation, mais est formé par des 

 cellules plus volumineuses . et plus claires qu' au póle supérieur; 

 l'autre, profonde, fait corps avec les elements du plancher. C'est ce 

 clivage. que j'ai appelé "clivage gastruléen", c'est lui, qui caractérise 

 la première phase de la gastrulation, parce qu'il ainène, en dessous 

 de l'équateur de l'oeuf, la formation d'un feuillet enveloppant et 

 d'une masse cellulaire enveloppée, d'un ectoblaste et d'un endoblaste." 

 And farther on : "lorsque ce clivage est achevé, il est clair, qu'a 

 sa limite inférieure, l'ectoblaste et l'endoblaste se continuent l'un 

 dans l'autre, comme Ie faisaient antérieurement la voute et Ie plancher 

 de la eavité de segmentation." 



This line of continuity Brachet calls "blastopore virtuel" ; after 

 a short time this virtual blastopore is converted into a real blastopore 



1 ) Archives de Biologie Tome 19 1902 and Anatom. Anzeiger. Bd. 27 1905. 



2) Anat. Anzeiger Bd. 27, p. 215. 



