JEXXIXGS: DEVELOPMENT OF- ASPLAXCHXA HEPJilCKII. 57 



occupied by d^-*, the sagittal plane passing through this cell forming a 

 plane of symmetry for all. The median plane thus indicated coincides 

 ■with that already defined by the line separating the quadrants A and B 

 anteriorly, and the boundary between the cells d^-^''- and d^-^'^ posteriorly ; 

 it passes through the animal pole and the blastopore. 



This movement of the entodernial blastomeres is of course simply a 

 continuation of the rotation inaugurated at the passage from the four- to 

 the eight-cell stage. The ectodermic cells continually withdraw their 

 deeper parts and increase their surface area at division ; in this way cells 

 are continually forced in at the blastopore. These press upon the anterior 

 and ventral aspects of the entoderm cells, forcing them backward, as 

 already described. The pressure is greatest in the median region, so 

 that the anterior or ventral ends of the cells on the two sides of the 

 median plane are forced apart, the axes of the cells become oblique to 

 the long axis of the embryo, and the oblique position of spindles shown 

 in Figure 83 results. 



Beyond this point it is impossible to trace the development cell by 

 cell. In Asplanchna there is a period, intervening between the stage to 

 which it is possible to trace the cleavage step by step (about 120 cells) 

 and the stage of recognizable differentiation of organs, during which the 

 cells divide and become very minute. The cells probably reach the 

 numjoer of from 250 to 500, and the process of extension of cells and 

 consequent " rotation " and invagination of parts of the embryo continues. 

 A sagittal section of the embryo at about the time of the beginning of 

 differentiation of organs is shown in Figure 84. It thus becomes 

 impossible to trace the fate of individual blastomeres, or even, except in 

 a most general way, the fate of the different regions of the embryo during 

 the later portion of the cleaving. From the small size of the adult roti- 

 fer, I had hoped that it would prove to be a favorable object for an exact 

 study of the cytogenetic history of organs ; in Asplanchna this turns out 

 not to be the case. But, on the other hand, for a study of the factors in 

 the early developmental processes it has shown itself well fitted. 



My study of the processes in the early development of Asplanchna there- 

 foi'e closes with the stages shown in Figures 79 and 83 (Plates 9 and 10). 



For critical comparison of my observations wdth those of Zelinka and 

 other workers on the development of the liotifera, the reader is referred 

 to Part Second. 



I shall now proceed to a discussion of the bearing of the foregoing 

 observations upon the problems already pi'oposed, as well as upon other 

 related subjects. 



