The Development of Ischnochiton. 623 



terminating in an enlargement (Fig. D, b). As invagination proceeds 

 and the blastopore narrows this furrow disappears. It is said however 

 that this same furrow reappears when the blastopore commences to 

 shift to its permanent position. 



In Figs. 35 and 39 is found just such a furrow as Metcalf 

 describes. It extends from the blastopore out towards the velum and 

 terminates in an enlargement. Also at its commencement it includes 

 the two products of the fourth quartette cell (mesoblast). 



Since in Chiton these two cells lying in the groove are in all 

 probability mesoblast, it follows that the furrow does not extend 

 anteriorly but posteriorly, and the furrow between the blastopore and 

 the prototroch appearing when the former commences to shift is not 

 the same one but another which lies on the anterior side of the 

 embryo as is found also in Ischnochiton. At such a stage landmarks 

 are not easily discovered and therefore it might readily be supposed 

 that these two furrows, the posterior and anterior, occur in the same 

 band of cells, but if a more careful study of the cell lineage is possible 

 I feel confident that Chiton and Ischnochiton will exhibit the same 

 phenomena. 



KowALEVSKi ('82) in a brief account describes some highly unique 

 changes in the development of the blastopore of Chiton jwlii. I quote 

 Balfour's ^) succinct abstract at this point. The embryo has prob- 

 ably reached a stage somewhat similar to that in Fig. 35 and of the 

 following changes he says: "In the succeeding developmental period 

 the blastopore which has so far had the form of a circular pore at 

 the posterior extremity of the body, undergoes a series of very 

 remarkable changes. In conjunction with the gradual elongation 

 of the larva it travels to the ventral side and is prolonged 

 forwards to the velum as a groove. The middle part of the 

 groove is next converted into a tube which opens externally in 

 front and posteriorly communicates with the archenteron. The walls 

 of this tube subsequently fuse together, obliterating the lumen, and 

 necessarily causing at the same time the closure of the blastopore. 

 The tube itself becomes thereby converted into a plate of cells on the 

 ventral surface between the epiblast and hypoblast." 



The nervous system is believed by Kowalevski to arise from a 

 portion of this plate. In the Molluscs the pedal nerve cords arise 

 posterior to the mouth" and if the groove in Ch. polii does form 



1) Comparative embryology, V. 1, p. 255. 



