No. I.] THE EMBRYOLOGY OF CREPIDULA. 105 



again, especially with the appearance of bilateral symmetry. 

 In all such cases, the direction of cleavage and the consequent 

 position of cells is due to something other than the alternation 

 of cleavage, surface tension, or intercellular pressure. For the 

 present, therefore, one is justified in assuming that these pecu- 

 liarities in the direction of cleavage, and in the position of result- 

 ing cells, is the result of intrinsic rather than of extrinsic causes. 



This conclusion holds true with especial force in the study 

 of the relative sizes of the cells composing the cross, and other 

 adjacent structures. The very small size of the tip cells has 

 been emphasized by Heymons and myself ; upon their size 

 depend in part the shape and structure of the entire cross. 

 The more rapid divisions and consequent smaller size of the cells 

 of the anterior, the right, and the left arms, as compared with 

 those of the posterior arm, the great size of the turret cells 

 and of the anterior cell plate, — all these contribute to the most 

 characteristic features of the cross ; and yet the known mechan- 

 ical conditions of cleavage are wholly unable to explain them. 



On the other hand, it is certain that the size of many of 

 these blastomeres can be directly correlated with their pro- 

 spective functions {e.g., the cells of the posterior arm, the turret 

 cells, the apical cells, the anterior cell plate), and while it is 

 not possible at present to explain all the characters of the cross 

 and the rosette series in this way, a strong presumption is cre- 

 ated that these structures, like teloblastic rows of cells, are 

 to be explained as a precocious development of certain parts. 

 The cells of the right and left arms and their derivatives have 

 apparently the same fate, while the destiny of the cells of the 

 anterior and posterior arms differs from that of the transverse 

 arms and from each other. The most obvious significance of 

 the cross is that its cells represent the protoblasts of certain 

 structures which are ultimately to lie in the median and trans- 

 verse axes of the larva. The identity of the right and left arms 

 is correlated with the fact that the organs are identical on the 

 right and left sides. For a long time the anterior arm is iden- 

 tical with the right and left arms ; in its later stages, however, 

 it becomes slightly different, and in the end gives rise to some- 

 what different organs. From an early stage the posterior arm 



