292 TK G. VanN^ame — Enihryolof/i/ of Eii.^ii/J.ocJiiis. 



The Second and Later Cleavages. 



The conditions in the case of these planarians are particularly 

 unfavorable for tracing the cleavage centrosomes through from one 

 division to the next, for a stage intervenes when the aster raj^s of 

 the preceding spindle have disappeared or practically so, and the 

 new rays have not as yet developed. All that remains is an area of 

 granular protoplasm lying against tlie nucleus, very similar in 

 appearance to that found in the earlier stages already described, and 

 there are generally black staining spots or granules present, making 

 it impossible to recognize the centrosomes. By the time the new 

 aster rays are visible these lie, if not on opposite sides of the 

 nucleus, at least widely apart, and no central spindle can be seen 

 connecting them. At an earlier stage such a spindle may exist, 

 but the specimens upon which this opinion is based are too obscure 

 to be of much value in proving it. 



The chromosomes of the second cleavage spindle are formed in the 

 same way as those of the first, except that they are all derived from 

 the single nucleus instead of from the two pronuclei. A part of the 

 niiclear reticulum begins to stain deepl}^, the nucleoli disappear and 

 the membrane is dissolved as in the case of the pronuclei. In the 

 subsequent phases the second cleavage spindle resembles the first. 



As already mentioned, the second cleavage is somewhat unequal, 

 and the third is still more so. Mitosis in the later stages seems to 

 follow the same rules, but as the cells become smaller its observation 

 presents more difficulties. In the late stages the centrosomes and 

 centrospheres are larger and the spindle proportionately shorter and 

 wider. This is already quite apparent in the third cleavage spindle 

 shown in Fig. 29. 



Later Development of the Embryo. 



The development goes on both by the division of the small cells 

 and by the budding off of more small cells from the large ones, 

 but I have not attempted to work out the cell-lineage. Eustylochus 

 is a most unfavorable animal for this kind of work because of the 

 small differences in the size of the blastomeres, in which it is 

 unlike some other polyclads, especially Discocells, described by Lang 

 (18). The slowness of development and the opaqueness of the 

 blastomeres add to the difHculty. 



The result of the succeeding divisions is tliat a ca|) or envelope of 

 small ectoderm cells extends further and further from the animal 



