RELATION BETWEEN AMITOSIS AND MITOSIS. 1 45 



nuclei in stages like those shown in Plate VI. have never been 

 observed. The smaller nuclei are, without doubt, dividing more 

 rapidly than the larger, and we are probably justified in conclud- 

 ing that amitosis occurs in those regions of the egg where division 

 is most rapid, while mitosis is found, when it occurs at all, among 

 the nuclei which are dividing more slowly. 



In Figs. 28 and 30 (PI. VI.) two cases of mitosis are figured. 

 In these two cases, in fact in every case of mitosis observed dur- 

 ing later cleavage, the spindle lies within a blastomere which is 

 bounded on all sides by a distinct membrane. This is particu- 

 larly well shown in Fig. 30 where the blastomere undergoing 

 mitosis forms a spherical mass with distinct membrane in the 

 midst of the egg-syncytium. This fact, like others mentioned in 

 preceding papers, 1 seems to indicate that conditions in regions 

 where mitosis occurs are widely different from those in which 

 nuclear division is amitotic. Evidently some physical or chemical 

 condition is present in the region about the mitotic spindle in 

 Fig. 30 which determines the formation of a cell-membrane about 

 a certain mass of the cytoplasm. It is not impossible that the 

 membrane may be a cOagulation-product resulting from differ- 

 ence in electrical condition of the colloids in the two regions. 

 Suggestions of this nature have been made by various authors, 

 both as regards nuclear membranes and cell-membranes of this 

 character. 



As is evident from the figures, the embryos are quite irregular 

 in shape. As cleavage proceeds, however, elongation commonly 

 occurs, but whether this elongation is a mechanical deformation 

 resulting from pressure of the uterine walls as the egg is forced 

 through narrow openings, or whether it is a typical feature of 

 morphogenesis is not certain. Such cases as Fig. 32 (PI. VII.) 

 in which one end of the embryo is drawn out into a sharp point 

 seem to indicate mechanical deformation. 



Even in these later stages of cleavage it has not been possible 

 thus far to discover any certain basis for orientation of the embryo. 

 In many cases one large yolk-bearing blastomere is found at each 

 end of the elongated embryo (Fig. 27, PI. VI.). In other cases 

 only one such blastomere appears (Figs. 30, 31, PI. VI.). My 



1 Child, Biol. Bull., XII., 2, 3, 4, 1907. 



