6 Growth in length of the Vertebrate Embryo 



We shall find that in the Anamnia the experimental evidence 

 supports this view. 



3. Then again it is possible theoretically to hold the view that 

 the main axis of the embryo is really formed by the coalescence of 

 the lips of the rim each half side of the rim representing one-half 

 of the embryo. This view is known as the concrescence theory. 



Since the rim is regarded as the lip of the blastopore (the 

 uncovered yolk as a plug filling up the blastopore) then, according 

 to the concrescence theory of vertebrate development, the blasto- 

 pore is supposed to fuse by the concrescence of its lateral lips, 

 each lip giving rise to the corresponding half of the embryo, 

 Fig. 3 C, and the blastopore finally closing. 



According to this theory the relation of the main axis of a 

 Vertebrate to the blastopore is very different from that which it 

 has if we adopt alternative No. 2. According to the concrescence 

 theory the relation of the main axis to the old blastopore of 

 coelenterate days the gastraea stage is the same in Vertebrates 

 as it is in Annelida and Arthropoda. If we adopt No. 2, the 

 main axis is in a direction at right angles to what it is in Annelida 

 and Arthropoda. 



This theory of concrescence started by His in 1874 has been 

 brought forward time after time by embryologists in newer and 

 more subtle forms. It has received support from many authorities, 

 and has been vigorously opposed by others. 



GROWTH CENTRES. 



Quite apart from the question of concrescence or the heresy of 

 concrescence, as I would prefer to call it there is another question 

 of considerable importance, and of especial interest with respect 

 to the difficult question of gastrulation and the formation of the 

 germ layers, the ectoderm and endoderm. 



If we look at an early stage of the trout embryo we see a 

 perfectly circular disc of cells. Later the rim thickens along the 

 border which will ultimately be the posterior and dorsal part of 

 the embryo and rim. But what about the thickened area just 

 within this rim? Fig. I, C. Is that derived from the rim, or 

 is it formed so to speak in situ on the blastoderm ? And what 



