Theories with respect to Amphioxm 51 



Growth in length occurs by the activity of the deuterogenetic 

 mass, which as in the Anamnia is most active dorsally, giving rise 

 to the bulky dorsal region of the body. 



The Lower Chordates. 



Among the lower chordates the question of growth in length 

 has been considered with respect to Amphioxus by a number 

 of investigators. Hatschek originally described growth in length 

 as being due chiefly to the activity of two large cells which he 

 termed polar cells, and from which he supposed all the hinder 

 mesodermal segments were formed. Others, Wilson, Morgan, etc., 

 have shown that these polar cells were mythical and they described 

 growth in length as due to a general mass of proliferating tissue 

 as in the higher chordates. 



Lwoff, Cerfontaine and others have also worked at the subject. 



Hatschek and Cerfontaine have maintained that growth in 

 length is due to concrescence of the dorsal lips of the blastopore 

 in precisely the way that we have shown to be quite untenable 

 for the Teleostean and Elasmobranch fishes. 



Lwoff described the dorsal wall of the hinder part of the embryo 

 as forming from the dorsal lip by a process of inrolling of the 

 ectoderm, this giving rise to what he called an ecto-blastogenetic 

 plate. 



MacBride has more recently gone into the matter and has 

 come to the conclusion that there is no such thing as a concrescence 

 of the dorsal lips of the blastopore, but that increase in length 

 is due to a growing point exactly comparable to that which I 

 have already described as a deuterogenetic centre, due to the 

 general activity of the lips of the blastopore. MacBride however 

 considers that the closing of the blastopore is very largely due 

 to the growth upwards of the ventral lip of the blastopore; a 

 condition which does not fall very well into line with the method 

 by which the Frog's blastopore closes. Here, as in the anamniate 

 craniates, the more active part of the blastopore rim is the dorsal. 



This fact according to MacBride becomes evident very early, 

 even while gastrulation is in progress (Fig. 8). 



In the same way as in the higher chordates the ventral part 



42 



