192 THE ANCESTRY OF VERTEBRATES 



neurotroch of Annelids and the medullary tube of Verte- 

 brates, and the posterior part of the circular rudiment of 

 the neural plate surrounding the wide open blastopore 

 might be compared with it. In Amniotes ihe primitive 

 streak, along which the blastopore moves backwards, is 

 then distantly comparable to the neurotroch, since here too 

 we have to deal with the coalesced lateral borders of the 

 blastopore. 



In fig. 36^ has been indicated how in Craniates the 

 praechordal cerebral plate {h.pl.) is added to the epichordal 

 neural plate (m. pi), while in Acrania the condition shown 

 in fig. 36/ persists. 



Summary. — From the foregoing considerations on the 

 gastrulation of Chordates the following conclusions result: 



1. All that sinks beneath the surface represents the pri- 

 mary endoderm or hypoblast, and that which remains 

 at the surface is the ectoderm or epiblast (contra 

 LWOFF). 



2. That which formerly has been always considered 

 as the blastopore is indeed the blastopore, not a 

 "notopore" (HUBRECHT) or a "somatopore" (De LangE) 

 which would be only a very little part or the rest of 

 the blastopore. 



3. The phenomena occuring during the contraction of the 

 blastopore border must be called gastrulation, not 

 "notogenesis" (HUBRECHT> or "somatogenesis" (De 

 LANGE), following after the gastrulation. Truly, during 

 gastrulation the embryo of Amphioxus assumes an oblong 

 shape, which proves that at this moment the growing out 

 of the segmented soma has already begun, as is shown also 

 by the circumstance that only a little later a whole series of 

 mesodermic segments appears. If, however, we call the 

 growing out of the segmented soma in Annelids "somato- 

 genesis"', then we must say that in Vertebrates the "soma- 

 togenesis" interferes with the gastrulation and does not 

 come after it In this sense I propose to use the term 

 "somatogenesis" in future ASSHETON (1894) was right in 

 distinguishing two growing processes of which the latter, 

 called notogenesis by HUBRECHT and somatogenesis 

 by De Lange, is connected with the longitudinal 

 growth of the embryo, but he did not make out the 

 correct relation between the two and the way in which 

 they interfere mutually. 



