232 O. C. GLASER. 



If the lining of the entire embryonic digestive tract were tempo- 

 rary, one should be able to find reserve elements from which at 

 a later stage the definitive entoderm might be derived. Care- 

 ful search has failed to reveal such cells. Even granting that 

 such reserve cells do indeed exist, but that they are not suffi- 

 ciently well characterized to attract attention, there are no regions 

 in the entoderm in which amitosis is absent, and the assumption 

 that there are reserve cells involves, of necessity the belief that 

 the definitive entoderm comes from cells like those described and 

 figured. Since there are constant histological differences between 

 the nuclei in the two regions under discussion, and further since 

 the disintegrating cells are very definitely restricted, they can be 

 eliminated from the field of inquiry by tracing them to their 

 posterior limits and considering only cells well back of this 

 boundary. 



A third difficulty was encountered when it was found that not 

 only is it impossible to cut mitotic figures and amitotic nuclei 

 serially into an equal number of sections, but they cannot even 

 be sectioned in an equal variety of planes that will reveal their 

 true character. Actual measurements, as well as experiments 

 with models representing direct and indirect nuclear division, 

 show that when nuclei are equal in volume, one in anaphase can 

 be cut in many more planes that will reveal its true mitotic char- 

 acter, than an amitotic nucleus of equal mass. In fact in very 

 late stages of amitosis, stages in which the daughter nuclei are 

 connected with one another by very small or very attenuated 

 bridges, only planes passing through the long axis of the dumb- 

 bell shape will exhibit the true relation of the lobes. Since the 

 amitoses probably take place in all possible planes, the error due 

 to the above factors is no doubt a considerable one. 



A fourth difficulty needs to be considered, namely, the possi- 

 bility that the larvae studied were abnormal. To eliminate errors 

 due to this source I used more than one embryo in each of the 

 stages represented in Fig. 9, except the first two, of which no 

 greater number was available. Since the argument, as the sequel 

 will show, does not hinge on individuals, but on a comparison of 

 the first half of the developmental period considered, with the 

 second half, the scarcity of early stages is compensated for. Thus 



