34 BIOLOGICAL LECTURES. 



that they suggest a number of highly interesting problems for 

 future research. As regards the third quartet I was unable to 

 find in Leptoplana any evidence that it gives rise to mesoblastic 

 elements such as we should expect to find in the Turbellaria in 

 view of the formation of ectomesoblast from this quartet in 

 P/iysa, Planorbis, Podarke, and probably in Aricia. As far 

 as I could find, the third quartet gives rise only to ectoblast 

 cells at the lip of the blastopore (Fig. 4), and Lang's results seem 

 to me inconclusive on this point. Only renewed researches 

 can determine whether this difficulty be real or only apparent. 

 In the mean time it would be well not to lose sight of the fact 

 that the polyclades cannot, of course, be the actual ancestors 

 of the annelids and mollusks, and that the cleavage in the 

 former may differ very considerably from the common ancestral 

 type. A natural hypothesis is that in the ancestral mode of 

 development all of the first three quartets gave rise both to 

 ectoblast and to mesoblast, and that in all the existing forms 

 the mesoblast formation has been lost in the first quartet and 

 variously reduced or entirely suppressed in one or both of the 

 two succeeding quartets. I think, therefore, that we need not 

 hereafter be surprised to find the formation of ectomesoblast 

 from more than one of the first three quartets, whether in the 

 Turbellaria or in the higher forms. 



It is when we attempt to bring the foregoing considerations 

 into relation with the history of the fourth quartet in annelids 

 and mollusks that we arrive at a far more serious difficulty ; but 

 we can hardly regret a difficulty that is so suggestive of further 

 research. In the polyclade the fourth quartet is relatively very 

 large, the basal quadrants being correspondingly reduced (Fig. 4). 

 All of the eight cells formed give rise, as far as known, to ento- 

 blast only. In the annelids and mollusks, on the other hand, 

 only three cells of this quartet anterior, right, and left are 

 purely entoblastic, while the fourth, or posterior, cell ("d 4 ") 

 divides into symmetrical halves to form the " primary meso- 

 blasts," or pole-cells, from which arise the two mesoblast-bands 

 characteristic of these groups (Fig. 2, A). Now, in comparing 

 this mode of development with that of the polyclade, we must 

 choose between the following alternatives. Either the meso- 



