80 bulletin: museum of compaeative zoology. 



VI.) with Wilson's Plate XV. Fig. 23 shows at once the great difference 

 in the shape of the egg, and the mechanical environment of the cells 

 under consideration. The evidence in this case, therefore, seems to 

 point to some other force than that of mechanical condition as the 

 determining cavise of this remarkable agreement. 



The intimate association of the spiral and bilateral types of cleavage, 

 and also the prevalence of spiral cleavage in those animals possessing 

 precociously developed larval forms, in which bilateral symmetry and 

 histological differentiation are early impressed upon the cleaving ovum, 

 suggest that the cause of spiral cleavage does not lie entirely in the 

 external mechanical environment of the cells, but is, in part at least, 

 to be referred to the same " morphogenic force " which produces the 

 bilateral symmetry of the embryo and the adult. That the ultimate 

 fate of cells exercises a profound influence upon their cleavage is well 

 shown in the precocious cleavage of the mesoderm quadrant in Nereis 

 and Unio, and of the teloblasts of the larval excretory organs in 

 Umbrella. It may be that in like manner spiral cleavage itself is 

 but a manifestation of precocious development of the organism as a 

 whole. 



It is also difficult to explain the alternation of spirals by the mechan- 

 ical conditions attending their formation. A glance at the tables of 

 cleavage which I have given will quickly suggest that, although we have 

 the same spiral in a given division of any generation in all the eggs 

 having spiral cleavage, the conditions under which the spiral is formed 

 in the eggs of different animals are by no means identical. The chrono- 

 logical sequence of the division of quartets in different eggs is not the 

 same ; neither is the distribution of the yolk, either in quantity or 

 quality. The spirals however are always identical wherever they occur. 

 These external mechanical conditions have doubtless a profound influ- 

 ence, but are they the only or the prevailing ones'? If we predicate 

 this, we must maintain that the resultants of these variously combined 

 mechanical influences are identical in all cases of identical spirals. Be 

 the cause of the spiral what it may, the intei-nal conditions of nuclear 

 division seem to be correlated with the alternation in direction in suc- 

 cessive generations. In an unimpeded field of action, the division and 

 subsequent equal migration of the two daughter centrosomes would 

 necessarily produce a series of cell divisions at right angles to one 

 another. This element is doubtless one of the factors in that field of 

 complex activities, the cleaving ovum. 



