GROWTH IN LENGTH OF THE FEOG EMBRYO. 239 



ventral, and so the dorsal part of the embryo is developed 

 more quickly than the ventral. 



I believe the true way of regarding this area of secondary 

 proliferation, both in the rabbit and in the frog, is as a single 

 area, whether circular, annular, or linear, whose sole function 

 is the addition of cellular units to the posterior end of the 

 previously existing embryo. Its form is tlie result of secondary 

 or ontogenetic causes. 



The exact line of demarcation is not easy of determination. 

 Very careful marking of the dorsal lip might give it as far as 

 the nervous system is concerned, but organs, no doubt, change 

 their relative position somewhat as they develop. The brain 

 is certainly thrust somewhat forwards. I brought forward 

 evidence to show that in the rabbit this point was about the 

 level of the first mesoblastic somite. It is, at any rate, 

 possible that metameric segmentation may be directly due to 

 this process of elongation. It seems always to be closely 

 connected with it. If so it may be due to this, that the 

 anterior mesoblastic somite of the frog is the smallest, and 

 each for succeeding five or six becomes longer dorso-ventrally 

 than its preceding neighbour. 



For upon my suppositious of the non-concrescence of the 

 blastoporic lips, and of the unity of the nature of this area 

 of secondary proliferation, then, since the first part of this 

 proliferating area to be formed is that part adjoining the 

 dorsal surface, those parts in the mid-dorsal line, e. g. neural 

 plate, will be the first, and at first the only part of the 

 embryo to receive additions from the proliferating area. As 

 the lateral lips of the blastopore are formed, more and more 

 of the lateral plates of mesoblast will receive additions, so that 

 in this way it is possible that the gradual increase in size of 

 the first six mesoblastic somites in the frog may be connected 

 with the gradual development of the area of proliferation. 



Every one is agreed that there is a certain part of the neural 

 plate formed on an area anterior to the first commencement of 

 the blastopore lip. The point in discussion is to what extent 

 does this pre-blastoporic formation exist ? 



