60 Growth in length of the Vertebrate Embryo 



end remains open as the mouth, and the anus breaks through at 

 the posterior end of the seam. 



Balfour says that Stossich and also Willemoes-Suhm state that 

 in one species of Serpula the part of the blastopore remaining 

 open is the anus. 



In Oligochaeta the blastopore becomes the mouth. 



This can all be easily rendered intelligible by assuming that 

 in evolution the blastopore elongated and then by closing centrally 

 gave rise to the mouth and anus. 



Growth in length in this case has been due to the persistence 

 of part of the blastopore rim as the deuterogenetic centre. But 

 which part? Clearly not the mid-dorsal as in Chordates. It is 

 probably caused by the retention of a part of the coalesced lip. 

 This is the part which has been sometimes named the teloblast. 

 It gives rise to the trunk and the so-called coelomesoblast ; and as 

 it is situated between the mouth and anus the effect of its activity 

 is of course to separate these two openings more and more. 



In Hydroides uncinatus (Eupomatus), Fig. 33, the cells which 

 give rise to the coelomesoblast from which the greater part of 

 the trunk mesoblast is derived have their origin in the lips of the 

 blastopore, as Shearer has shown, or as Tread well has shown in 

 Podarke, near to the future posterior end. 



In many Annelids (and Molluscs) there are other mesodermal 

 cells which are not derived from these two blastopore lip cells, 

 but either from the ectoderm or other part of the endoderm. 

 These form the so-called larval mesoblast or ectomesoblast. 



If we use the same terminology as that which we have used 

 with regard to the Chordates, then clearly the ectomesoblast is 

 protogenetic mesoderm, and the coelomesoblast in these forms is 

 deuterogenetic. The growth in length is due to deuterogenesis but 

 it is due to the persistence of a different part of the deuterogenetic 

 ring, and the result is a lengthening in the plane of the blastopore 

 in Annelids, and not at right angles to it as in Chordates. 



Moreover in Annelids both mouth and anus have had their 

 origin in the gastrula coelenterate mouth, whereas in Chordates 

 only the anus has been derived therefrom, Fig. 31. 



The group most closely allied to the Annelida is undoubtedly 

 the Arthropoda. In these there is plenty of indirect evidence 



