A STATISTICAL STUDY OF MITOSIS AND AMITOSIS. 239 



II. actual counting of complete sections in an approximately 

 comparable region, viz. : midway between the extremities of the 

 long axis, gave 20 as the number of cells, whereas for stage III., 

 15 was the average of five incomplete sections taken near the 

 plane of the equator. It is practically impossible to secure entire 

 sections through larvae in this condition on account of the thin- 

 ness of both the body-wall and the wall of the digestive tract, 

 neither of which is thicker, in many regions at least, than the 

 pellicles around the ingested food-ova. In the sections from 

 which the particular estimate now under consideration was made, 

 one third of the circumference was incomplete ; as there were 

 fifteen cells in the other two thirds, I assumed that the torn 

 region represented a distance which in the entire embryo was 

 covered by five cells, an assumption justified by the study of 

 other sections. 



A comparison of stages II. and III. may suggest at first sight 

 that the younger embryo should have fewer cells than the older 

 one, since the latter contains so much more material than the 

 former. Professor Osborn says that there are not enough cells 

 in stage III. to enclose the food-ova. This however is a mis- 

 take ; there are enough cells, only in order to " cover the ground " 

 these are stretched almost beyond belief. Indeed the elongations 

 are frequently as attenuated as the delicate projections so char- 

 acteristic of mesenchyme cells. 



The number of entoderm cells in stages IV. and V. was deter- 

 mined in complete sections, for the body -wall as well as the wall 

 of the digestive tract have thickened so much in these older 

 embryos that it is a comparatively easy matter to section them 

 without injury. As the table shows, 57 cells is what I found in 

 the younger of the two oldest stages and 93 cells in the oldest of 

 all of those considered. 



Granting for the sake of argument that the number of ento- 

 derm cells in the earlier stages is twice as great as my deter- 

 minations indicate, the later stages would still show double the 

 number of cells in corresponding regions. Fig. 2 shows that 

 such an error is impossible. I am certain that the figures actu- 

 ally given are much nearer the truth and that instead of having 

 twice the number of entoderm cells, the later stages of the de- 



