JENNINGS : DEVELOPMENT OF ASPLANCHNA HERRICKII. 67 



Wheeler ('95, p. 309) states in regard to the first cleavage spindle of 

 Myzostoma : " lu Myzostoma the spindle does not conform to 0. Hert- 

 wig's law, but always lies at right angles to the long axis of the often 

 very narrow protoplasmic pillar of the egg." 



Castle ('9G, p. 250) states that in the division of the entoderm cells 

 of Ciona the spindles in certain cases lie in the short axes of tlie cells, 

 even when a shifting of the asters from a previous position in the long 

 axis must have occurred to bring about this condition. Castle states 

 that no mechanical explanation of this phenomenon offers itself, though 

 he holds that " other things being equal, it is true that the spindle arises 

 in the longest axis of the cell " (p. 231, note). 



In the decapod crustacean Virbius, according to Gorham ('95), the 

 egg is ellipsoidal in form, and the first cleavage spindle may occupy the 

 long axis, or be more or less inclined to it, or may even be nearly at 

 right angles to it. 



The cell divisions in the germ bands of Crustacea as described by 

 Bergh ('95), in which the spindles take the same direction for many 

 cell generations, should be added here (see also McMurrich, '95) ; though 

 the evidence from these must be weakened in the eyes of the upholders 

 of Hertwig's law by the fact that before division the cells " wachsen nur 

 in der "VYeise, das ihre Langsdurchmesser dem Querdurchmesser ziemlich 

 gleich wird und dann tritt die Theilung ein." 



The positive evidence upon the question from observations of normal 

 cell division is thus rather scanty, though doubtless some additions 

 might be made by a further study of the litei'ature. 



From both experimental evidence and observation of normal division 

 the only conclusion possible is, that in some organisms the spindle does 

 take a position in the greatest axis of the protoplasmic mass, apparently 

 without regard to other factors, while in other cases the position of the 

 spindle is determined by other factors, without regard to the form of 

 the cell. 



The i-esult is at first thought not very satisfactory, but this is not the 

 only organic phenomenon with regard to which such a conclusion must 

 be drawn. A few examples will make this clear. 



Stahl ('85) found that the direction of the first cleavage in the spores 

 of Equisetum is determined by the direction of the infalling rays of light. 

 A general statement of the effect of the direction of the light rays on cell 

 division would take a foi-m similar to the unsatisfactory conclusion above 

 given for the shape of the cells. 



If we leave cleavage and take up other growth phenomena, such as 



