54 Growth in length of the Vertebrate Embryo 



inch or more apart. This seems to prove that their separation 

 and hence the growth in length is due to a general interstitial 

 growth and not to a terminal area of cell proliferation. 



If deuterogenesis occurs it occurs all round the anus (or 

 blastopore) and there is no tail. The absence of tail is a very 

 important fact, for it is in the Hemichordata alone among 

 chordates that there is no tail. 



This is so important that it would justify the classification of 

 the Chordata into Chordata caudata, and Chordata ecaudata. 



Conclusion. 



To sum up we may say, both from anatomical observation 

 and from experiment, that growth in length of the embryo in all 

 anamniate chordates must be considered to be due to the origin 

 of a special area of cell production round the lips of the blastopore, 

 which converts the spherical form of the gastrula into the cylin- 

 drical form of the later embryo. Since this area of necessity comes 

 into being only after the gastrula is formed, we may recognise 

 two regions in the later embryo. One of these regions is the 

 direct result of the segmentation of the ovum culminating in the 

 gastrula, and having the general character of a radially symmetrical 

 form, and this on the whole is to be identified with the coelenterate 

 phase of evolution. The region of the body so arising has been 

 named the protogenetic region. The other region is that of later 

 origin produced by the proliferation of the lips of the gastrula 

 mouth. This has been called the deuterogenetic region. The 

 part formed from the protogenetic region includes the fore- 

 brain, probably also the mid-brain, the mouth, and possibly the 

 hind-brain as far as the origins of the 5th and 8th nerves, the 

 branchial region and heart and probably much of the gut. The 

 part formed from the deuterogenetic region comprises the re- 

 mainder of the hind-brain and spinal cord and tail, the whole of 

 the metamerically segmented mesoderm, and in the craniates the 

 renal organs. As regards the reproductive organs there is much 

 evidence to show that in the craniate chordates the actual germ cells 

 are, as one would expect, protogenetic in origin, but that they 

 migrate during development into the deuterogenetic region and 

 here undergo their maturation, and eventually find their way to 



