THE ANCESTRY OF THE VERTEBRATES 



641 



lengthening from in front backwards, and thus pushing its inner end and 

 the former blastopore under the neural body-wall to the caudal end of the 

 embryo. This is not the case, as Delsman himself points out. But, 



CAR0IOPOB£ 



rroMOOEuMi 



^e^ PR£0«AL CIUATED 



PROTONEPHRIOIUM- 



MESODERM 



Fig. 522. — A diagram of a trochophore larva. According to Delsman's theory of 

 the origin of vertebrates, the cardiopore (blastopore) of the trochophore larva is homolo- 

 gous with the neurenteric canal of vertebrates. In other words the relatively short 

 stomodeum of the annelid is stretched to become the neural tube of vertebrates. 

 (Redrawn after Hatschek.) 



he claims, processes which occur in early development serve to explain 

 why the neural tube does not elongate in this fashion. The backward 

 growth of the blastopore in vertebrate embryos involves the extension 



CHORDATE TROCHOPHORE ANNELID 



NEUROPORE( 



BLASTOPORE, 

 NEURAL TUBE\ ENTERON 



FOREGUT 

 MOUTH 



VnuS ! ''BLASTOPORE 



EYE 



^^-ANUS 



MOLLUSC 



Fig. 523. — Diagram illustrating the theory of Delsman that by change in the loca- 

 tion of the growth center a trochophore (A) may be converted into either a chordate 

 (B), or a mollusc (C), or an annelid (L>). 



of the neural plate caudad. According to Delsman this is not a process 

 of concrescence, as is sometimes assumed, but a true process of backward 

 growth. This precocious backward growth takes the place of the back- 

 ward elongation of the neural tube. The result is the same as if there 

 had been an elongation of the neural tube— the blastopore is carried 

 to the posterior end of the body, where it persists for a while as the 



