ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 589 



dilute solutions of acids, both inorganic and organic. After transfer 

 from the acid solutions into normal sea-water, the egg throws off a 

 typical fertilisation membrane, the germinal vesicle breaks down, and 

 maturation and cleavage follow. In successful experiments, which were 

 the rule, from 50-60 p.c. of the eggs developed into swimming larvae 

 that could scarcely be distinguished from normal trochophores of a 

 corresponding stage. The parthenogenetic development, in the majority 

 of cases, involves a strictly normal maturation, a normal cleavage, at 

 least in the early stages, and the usual processes of differentiation that 

 occur after fertilisation by sperm. Gastrulation takes place in the 

 normal manner, and the parthenogenetic larva possesses a digestive tract, 

 differentiated into mouth, oesophagus, stomach and intestine, and the 

 prototroch and apical plate, bearing the normal arrangement of cilia. 

 After maturation the egg-centre disappears, and the cleavage centro- 

 somes arise de novo, probably without division of a single primary 

 centre. When first seen, they lie on opposite sides of the egg-nucleus, 

 which becomes the first cleavage nucleus. 



Cell-division occurs mitotically throughout development, and division 

 of the nucleus is usually accompanied by cytoplasmic cleavage. The 

 number of chromosomes characteristic of the fertilised egg is not 

 restored, but the reduced number (12) is retained, and has been counted 

 repeatedly even in late stages. The rate of division is not so rapid, 

 nor so regular as in normal segmentation, and the parthenogenetic larva?, 

 although swimming vigorously at the bottom of the dish, do not rise to 

 the surface of the water. 



After exposure of the eggs to acid solutions, the polar bodies may 

 continue to divide mitotically and form a morula-like cluster of minute 

 cells, thus exhibiting an attempt at parthenogenetic development. In 

 some experiments the eggs extruded only one polar body, and in others 

 neither polar body was formed. In such cases, either one or both matura- 

 tion mitoses may take place within the egg, with the resulting formation 

 of resting nuclei, which probably fuse to form a cleavage nucleus. 

 In still other cases there is evidence for believing that the first matura- 

 tion spindle may directly become the first cleavage spindle, across 

 which the egg divides into equal or subequal cells. The numerical re- 

 lations of the chromosomes in these cases have not been definitely 

 determined. Eggs exhibiting these abnormalities of maturation give 

 rise to larvae indistinguishable from those of eggs which maturate 

 normally. An endless variety of abnormal cleavages, similar to those 

 described by others, have been observed. Such cleavages lead to the 

 formation of ciliated cellular structures which depart more or less 

 widely from normal embryos. Abnormalities of mitosis, as polyasters 

 and monasters, are not infrequent, and when nuclear division is not 

 followed by cleavage of the cytoplasm, chromosomes in excess of the 

 usual number (12) may be found in a single cell. Cytasters are either 

 absent or exceedingly rare, and cytoplasmic cleavage without preceding 

 nuclear division has not been observed. Amoeboid movements of the 

 egg are rare, and, when they occur, are not extensive ; " fusion phe- 

 nomena " are lackin<_r. 



Cell-division would seem to be a fundamental and essential factor in 



