LECTURE II 



We have been able to come to a fairly decisive conclusion with 

 respect to the growth in length of embryos of the Anamniate 

 Vertebrates. We have arrived at this conclusion chiefly through 

 anatomical observations and also from evidence obtained by actual 

 experiment upon certain fishes and Amphibia. 



We have seen that the production of undifferentiated cells 

 which go to build up the general form of the embryo occurs around 

 two centres of activity. The first may be said to be around the 

 protoplasmic centre of the fertilised egg, and to begin with 

 segmentation and to end (so to speak) with the accomplishment 

 of gastrulation this is the primary or protogenetic growth centre. 

 The second begins with the formation of the blastopore lip at 

 the commencement of gastrulation, and may be said to be 

 around a centre situated in the middle of the blastopore, for 

 the form which this centre of growth takes in the Anamnia is 

 annular. 



This is the secondary or deuterogenetic growth centre. It is 

 placed eccentrically to the primary growth centre and the effect 

 of its action is the growth in length of the embryo, i.e. deutero- 

 genesis. New tissue is added on to all previously existing tissues 

 with which it is in contact. 



Hence we are able to distinguish between protogenetic ectoderm 

 and deuterogenetic ectoderm, protogenetic part of the nerve tube, 

 and deuterogenetic part of the nerve tube, protogenetic part of 

 notochord, deuterogenetic part of notochord, and so on. 



The clear appreciation of this fact renders intelligible the differ- 

 ences said to occur in the method of formation of the notochord and 

 explains how it can arise either from endoderm or rnesoderm or 

 ectoderm. It does away also altogether with the idea of gastru- 

 lation occurring in two phases as Hubrecht, Keibel and some 



