62 Growth in length of the Vertebrate Embryo 



closes in the middle region but the ends remain permanently open 

 as mouth and anus. 



So in the Mollusca in some, as for instance, Ostrea, the blasto- 

 pore remains open as the mouth, in others, Pisidium, it remains 

 as the anus, in others, Teredo, it closes altogether. Such incon- 

 sistencies taken in conjunction with what has just been said in 

 regard to the Annelida and Arthropoda may well be interpreted, 

 as Balfour suggested long ago, as indicating that in the Molluscs 

 also the gastrula mouth has given rise in the course of evolution 

 to both mouth and anus. 



In the Mollusca the deuterogenetic activity would seem to 

 occupy a position corresponding to that of the Annelid and 

 Arthropodan group, and we can recognise protogenetic and 

 deuterogenetic mesoderm produced in a very similar manner, but 

 the deuterogenetic activity dies out much earlier and growth in 

 length is limited. 



The conditions of development are thus totally different to 

 those of the Chordata and Echinodermata in which the gastrula 

 mouth gave rise to anus only. 



In the Nemertean phylum the gastrula mouth would appear 

 to have become mouth only, and the anus is a new formation. 



It may be asked what about metameric segmentation? Is 

 the metamerism of the Annelid in no way homologous to that of 

 the Chordate? 



To this I would reply, no more so than the metameric segmenta- 

 tion of the Annelid is homologous to that of the fir tree. 



The similarity is one of analogy. The deuterogenetic regions 

 of the Annelid and of the Chordate show metameric segmentation 

 because in each case growth in length is due to the same physio- 

 logical cause, which is quite analogous to that which produces 

 growth of the stem of a fir tree. Axial organs can be indefinitely 

 produced in length without interfering necessarily with their 

 utility. Thus the intestine of an earth-worm, the notochord or 

 spinal cord of a vertebrate, or the pith of a fir tree, can be 

 indefinitely extended and yet retain their original functions. 



But we cannot imagine lateral organs like lateral blood vessels, 

 lateral nerves, lateral muscles, limbs, leaves on a stem, being 



