No. I.] THE EMBRYOLOGY OF CREPIDULA. 185 



The conclusion, therefore, is unmistakable that bilaterality 

 first appears in processes which lead to the formation of the 

 trunk and the elongation of the future animal, while the primi- 

 tive radial symmetry of the anterior quadrants, which is so long 

 preserved, is correlated with the fact that these quadrants give 

 rise largely to larval organs, most of which bear traces of radial 

 symmetry. 



(c) Significance of the Forms of Cleavage. 



The cause of alternating cleavages in general has been 

 very fully discussed by various writers, particularly by Sachs 

 {Physiology of Plants, ch. XXVII) and by Hertwig {Die Zelle 

 und die Gewebe, ch. VI). The latter author has presented, 

 in the form of two laws, the principles upon which alterna- 

 tions in division are based. These laws are : (i) the nuclear 

 spindle lies in the direction of greatest elongation of the proto- 

 plasm; (2) the division walls intersect the spindles and the 

 previous division planes at right angles. It is probable that 

 these principles are true in general, but they meet with many 

 exceptions in the development of most animals. The chief 

 objection to these laws is that they assume that protoplasm is 

 an inert substance which behaves during and after division like 

 so much clay. On the other hand, nothing is more certain 

 than that protoplasm has intrinsic powers, which are, at least 

 occasionally, capable of setting aside these mechanical princi- 

 ples : e.g., it is able to change its shape so that it may elongate 

 twice or a dozen times in the same direction, as is seen in most 

 cases of teloblastic growth; or the axis of the nuclear spindle 

 may lie in the shortest diameter of the protoplasm, and the 

 division take place apparently in the direction of the greatest 

 pressure (cf. McMurrich, '95' and '95^); or the division wall 

 may intersect the spindle obliquely (as I have observed in 

 several cases in Crepidula) ; or successive division walls may 

 intersect each other at any angle from 0° to 90°. The setting 

 aside of these as well as many other mechanical principles on 

 the part of living matter is due to the fact that protoplasm is 

 not soapsuds or oil emulsion, but something vastly more com- 

 plex than either ; and it gives evidence that in cleavage, as in 



