56 BIOLOGICAL LECTURES. 



adaptation in the larva than vice versa, but the adaptation in 

 all stages of the development is due to some common cause. 

 It is, of course, impossible to be blind to the fact that this 

 " explains " determinate cleavage only in a very limited sense. 

 But if the principle serves to restrict the too universal applica- 

 tion in the cleavage of the egg of the so-called " mechanical 

 laws of cell-division," much will have been accomplished. 



V. PROTOPLASMIC BASIS OF ADAPTATION IN CLEAVAGE. 



It is possible to explain variations in size, position, and rate 

 of cleavage of the cells, by the hypothesis of differentiation of 

 the cytoplasm into materials of different qualities and positions 

 (His, Whitman et al.}. 1 A much more difficult question is imme- 

 diately suggested, namely, How do these different substances 

 arise within the cell, and how are they distributed to definite 

 regions ? Are all of the differences that exist in the thirty-two- 

 celled stage, for instance, also to be found in different sub- 

 stances of the unsegmented ovum, or are they formed, wholly 

 or in part, during the cleavage, and, if so, how ? 



The following observations, selected from my work on the 

 fertilization and first two cleavages of the egg of Unio, seem to 

 throw some light on this difficult problem. The spermatozoon 

 enters always at the centre of the vegetative pole, and describes 

 a penetration-path that carries it to a point between the centre 

 and the periphery of the egg near the lower boundary of the 

 upper (animal) half. The penetration-path is marked by a 

 portion of the cytoplasm cleared of yolk-granules by the activity 

 of the sperm-amphiaster (Fig. 12). During its penetration the 

 sperm-head has formed some caryolymph, and hence is some- 

 what vesicular ; but, arrived at its region of rest, it contracts 

 into a clump of small chromatic elements. All this has taken 

 place during the formation of the first maturation-spindle, and 

 is completed before the cytoplasm of the animal pole begins to 

 protrude to form the first polar globule. The sperm-amphiaster 

 and visible penetration-path have disappeared by the time of 



1 A theory of polarization of the ultimate morphological units would afford 

 only a partial explanation of the differences in rate and direction of cleavage and 

 in the size of the cleavage products. 



