584 WILSON. [Vol. VIII. 



p. 599) though the bilaterality of the tunicate is still more 

 striking owing to the inequality of the second cleavage. It is 

 much rarer than the first variety, and only in a single instance 

 was I fortunate enough to follow it continuously in the living 

 embryo from the undivided ovum up to a comparatively late 

 stage (Figs. 13 to 18, which show the development of the 

 same embryo from the 8-celled to the 128-celled stages). 



c. Spiral form. (Figs. 31, 32.) In this type, which in its 

 pure form is very rare, the blastomeres have precisely the 

 arrangement found in the i6-celled stage of the annelids 

 (Diagram, p. 599) I have not seen the origin of the blasto- 

 meres in the living embryo, and the connections of the blasto- 

 meres in the figures are hypothetical. The preparation figured 

 leaves no doubt, however, that each of the primary macromeres 

 must have divided unequally in the true spiral form to form 

 the four secondary macromeres. The arrangement of the 

 eight products of the first group of micromeres is exactly that 

 which would result from a spiral division taking place in the 

 reverse direction from that of the macromeres. 



The three forms described are connected by a perfectly 

 graduated series of intermediate forms, all of which I believe 

 are capable of complete and normal development. 



Beyond the i6-celled stage I have not endeavored to trace 

 out in detail the variations of the cleavage, since they are even 

 more numerous than those of the earlier stages. 



In the i6-celled stage the embryo consists of eight equal 

 micromeres, (products of a^-d^), and eight macromeres, viz., 

 the primary larger central group of four {A-D) and the second- 

 ary smaller peripheral group {A^-D^). As development ad- 

 vances from the i6-celled to the 256-celled stage the general 

 history is as follows : A second, third, fourth and perhaps a 

 fifth group of micromeres, each of them eight in number, are 

 formed by four successive unequal divisions of the macromeres. 

 These micromeres, as well as the products of the first group 

 (rt,i b,^ c,^ d^) divide equally, and thus a blastula is produced 

 in which the size of the cells increases pretty regularly from 

 the upper towards the lower pole. Throughout these stages, 

 and even as late as the 5i2-celled stage (Fig. 48) most of the 



