238 



BIOLOGICAL LECTURES. 



Fig. 5. — Arenicola. Fifth cleavage viewed 

 from upper pole. 2d, the " first somato- 

 blast" already formed. 



has become so firmly established, even in spite of the fact that 

 the free respiratory and excretory surfaces of the cells have 

 been reduced, in consequence of its presence, to such an extent 

 that in some eggs a compensatory arrangement in the form 



of the intercellular cavities is 



necessary. 

 Hozv this 



process has oc- 

 curred we cannot explain at 

 present, but that it Jias occurred 

 I believe we must admit, and 

 that there is sufficient motive 

 for its occurrence I trust I have 

 shown. 



The next point which requires 

 investigation is the fact that, in 

 the great majority of cases, the 

 direction of corresponding cleav- 

 ages is the same, i.e., the second cleavage is laeotropic and 

 the third dexiotropic, and so on (Fig. 5). Why should this 

 constancy occur .-* Directly correlated with this problem is the 

 question as to the cause of the few cases of reversed cleavage 

 in gasteropods. 



, If we suppose the cleavage to be hereditary, the direction of 

 the early cleavages is no longer a matter of chance, and the 

 direction of succeeding cleavages would follow according to the 

 principle of regular alternation. This being the case, the assump- 

 tion is not at all unwarranted, that, of the two possibilities, one 

 has become established and the other has disappeared. What 

 factor determined which one of the two possibilities should 

 become the rule is beyond our knowledge, but it is certainly 

 a fact that in nearly all spiral cleavages the second division is 

 laeotropic and the third dexiotropic, etc.; and it is just as 

 certain that this constancy is not merely the result of mutual 

 pressures in groups of four, eight, or more blastomeres. 



The occasional appearance of reversed cleavage is explicable 

 as a variation from the normal type, - — the only variation possi- 

 ble with continued conformation to the spiral type, — or in some 

 cases it may be that this variation for some special reason has 



