258 BIOLOGICAL LECTURES. 



A bit of material is given off at one point, serving to fill up a 

 chink, as it were; a large cell appears at another point, etc., and 

 every cell in the complex aids in the postero-lateral growth 

 and concrescence of the plate, not simply because it was 

 formed in a certain position, but because it is of a certain 

 size. Some cells, like the turret cells of Crepidula, are small 

 when formed, but later increase greatly in size. In these cases 

 it seems probable that the appearance of a large cell at the 

 time when these divisions occur would interfere with the dis- 

 tribution of the ectodermal material. The later increase in 

 size may be connected with the role which the cell itself is 



to play, or may be another 

 means of aiding in the distribu- 

 tion of the material. 



But in many cases a great 

 difference in size is found among 

 the cells at the very beginning 

 of cleavage, as evidenced by 

 the extremely unequal division 

 in many forms (Fig. 25). This 

 difference continues to be ex- 

 pressed at various points during 

 Y\G.:i'i:—Arenicoia. Unequal two-cell thc Spiral .pcriod, uotably in the 



stage from upper pole. ^ ^ . r .^ r . 



formation or the first somato- 

 blast, 2d (Fig. 5), and the mesoblast, A^d, and, in a less degree, 

 in many other cells. These are the differences which have 

 especially attracted the attention of nearly all those who have 

 studied this form of cleavage. They have been quite generally 

 regarded as differential in significance. There is here undoubt- 

 edly an anticipation of, and provision for, the later stages of 

 cleavage, but, to me at least, it seems that, so far as morpho- 

 genesis is concerned, it is, at least usually, rather quantitative 

 than qualitative. Certain avioimts, rather than certain kinds, 

 of material are stored up in certain cells just where they will 

 be in position to produce by coordinated action the "desired 

 result." Thus a still further "condensation" of the process 

 of development is effected. Cells which are to serve as centers 

 of distribution of material, e.g., the first somatoblast, may exceed 



