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Hon in vertebrates, given by Hubrecht and Keibel and confirmed by 

 a number of other embryologists, this process in the teleostean egg 

 too must be revised and more sharply defined. 



In my former paper I was led to divide the process of gas- 

 trulation into two "phases", one by which the gut-entoderm is formed 

 and one by which chorda and mesoderm are differentiated. But now 

 I think the line must be drawn still sharper and the second phase must 

 be separated entirely from the process of gastrulation sensu strictiori, 



According to the definition given by Hubrecht gastrulation is a 

 process by which a gut-entoderm is differentiated from an ectodermic 

 layer, and thus the germ consists of two distinct layers. The process 

 of formation of chorda and mesodermic plates, which follows directly 

 on the process of gastrulation proper (notogenese Hubrecht) is a 

 secondary complication of the process, characteristic of the vertebrate 

 embryo. 



The most primitive mode of formation of the entoderm, according 

 to Hubrecht, is by delamination and not by invagination. But after 

 all it is chiefly the outcome, the formation of the two germ-layers, 

 that is of interest. As soon as these two layers are formed and may 

 be distinctly separated from each other, the process of gastrulation 

 is finished. 



This is for example in amphioxus already the case at that stage 

 of development, in which the gastrula is cap-shaped, the two layers 

 (ectoderm and entoderm) are lying close against each other, the 

 segmentation-cavity has disappeared, but the blastopore still extends 

 over the entire breadth of the original blastula-vesicle. All the following 

 processes until the closure of the blastopore ("Rückenmund" of Hubrecht) 

 are notogenesis and lead to the formation of the back (chorda) and 

 of the mesodermic plates and to the closure of the gastrula-mouth. 



When we study again the teleostean gastrulation-process from this 

 point of view, we come to the conclusion, that in this ease the 

 process of gastrulation is ended as soon as the prostomal thickening- 

 has been formed, viz. at the beginning of the covering of the yolk. 

 At that moment the "Anlage" of the entoderm is clearly differentiated, 

 and the ectodermal cells begin to invaginate to form the chorda and 

 mesodermic plates ; the concentration of the cells towards the median line 

 begins, the long and slender embryo is formed out of the broad and 

 short embryonic shield. The blastula-cavity, in the cases in which it is 

 developed, has disappeared as such ; all the following processes, the 

 longitudinal growth of the embryo, the covering of the yolk by the 

 blastoderm ring, the closure of the yolk blastopore, belong to the 

 notogenesis and we are no more entitled to reckon these processes 



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