100 HOLMES. [VOL. I. 



its history are, in many respects, quite different, and the regu- 

 larity of its form becomes broken up before it reaches a cor- 

 responding degree of development. The cells of the posterior 

 arm enlarge and take part in the formation of the large head 

 vesicle. This enlargement of cells extends to the cells at the 

 center of the cross and finally continues forward to the proto- 

 troch ; thus the cap of cells lying above the prototroch is cut 

 into two groups of small cells separated by a median band of 

 large, clear cells extending from the head vesicle behind to the 

 prototroch in front. These isolated groups of small cells mul- 

 tiply rapidly and doubtless give rise to the eyes and cerebral 

 ganglia. 



The prototroch is formed from the trochoblasts previously 

 mentioned and the uppermost cells of the second quartette 

 , which form the tips of the arms of the cross. Possibly other 

 cells of the second quartette may take part in its formation ; it 

 is certain that the third quartette has no share in the process. 

 The cells of the prototroch, up to a late stage of development, 

 are few in number, and are arranged in a double row which 

 extends from the ventral side a short distance in front of the 

 blastopore to the head vesicle above. At an early stage they 

 acquire cilia which serve to rotate the embryo in the capsule. 



The head vesicle arises mainly from cells of the first quar- 

 tette lying on the posterior side of the egg. By the enlarge- 

 ment of these cells the apical pole is pushed forward through 

 an arc of 90 so that it comes to lie at the anterior end of the 

 embryo. An apparent effect of this process can be seen in 

 the cells lying in front of the blastopore ; many of these cells 

 take on a long, narrow form with their long axes transverse to 

 the median plane of the egg, thus giving the appearance of 

 having been flattened out by pressure from in front. 



The cells of the second and third quartettes become so num- 

 erous before any organs make their appearance that it is 

 impossible to trace the exact cell lineage of the structures 

 arising from them. All that can be determined is the cell 

 origin of the regions of the body wall from which certain 

 organs are developed. It is quite certain that the cells of the 

 posterior quadrant of the second quartette (derivatives of 2d) 



