458 R. M. PACE. 



difference in size and character between tlie cells of the oral 

 and aboral ectoderm is now seen to be very marked, and the 

 mesendoderm forms a solid mass entirely obliterating the 

 segmentation cavity. 



Summary. — To sum up the results obtained from the 

 foregoing study of the cell division : — 



Four equal cells — A, B, C, D — result from the primitive 

 cleavages. These four cells divide and give rise to eight 

 cells, arranged in two series, four small upper cells, and four 

 large lower cells, each of which series is destined to play a 

 distinct part in the subsequent history of the cell layers. 

 The four small upper cells, A^, B^, C^, D^, give rise only 

 to the aboral ectoderm. The four large lower cells, A|, B|, 

 C4, D|, give rise in part to the oral ectoderm, in part to a 

 tissue, which may for the present be best termed " mesendo- 

 derm." The cells from which the oral ectoderm is derived 

 are cut off from the four large oral cells by two successive 

 vertical divisions at right angles to each other. The four 

 large oral cells then divide a third time horizontally, and 

 the four upper products of this cell division pass into the 

 segmentation cavity, and there give rise to the primitive 

 endoderm, or rather, mesendoderm. Owing to the rapid 

 growth of the surrounding ectoderm, tlie four remaining oral 

 cells also eventually become enclosed within the segmentation 

 cavity, giving in all eight mesendodermic cells, which, by 

 subsequent division, eventually form a solid mesendodermic 

 mass. 



A true blastopore, which does not close until after the 

 formation of a considerable mass of mesendoderm, is present 

 during the stages referred to. 



The formation of mesoderm has not been actually observed; 

 it seems probable, hoAvever, that the mesoderm is derived 

 from the primitive mesendodermic mass at a later stage than 

 those hitherto dealt with, and this point will be further dis- 

 cussed (p. 467) when dealing with the degenerating larva. 



In view of the fact that the most essential of the larval 

 organs are developed from the oral ectoderm, it seems of 



