112 WHITMAN. [Vol. I. 



their general appearance. Throughout the embryonic period, 

 until long after hatching, these huge segments preserve their 

 individuality, only undergoing such slight changes in form and 

 position as are induced by the development of the germ-bands 

 and the epibolic expansion of the ectoderm. They contain most 

 of the food-yolk, which is utilized in the long post-embryonic 

 pseudo-larval period, and the elements which are to form the 

 mesenteron. 



The posterior macromere, x, on the other hand, undergoes 

 sucessive cleavages, resulting in the production of ten blastomeres, 

 or teloblasts, arranged in two bilaterally symmetrical groups, at 

 the posterior edge of the blastodisc (Diag. 4). 



The first cleavage-plane runs obliquely, beginning a little to the 

 left of the upper angle and taking a slanting direction towards the 

 right side, thus cutting off (^x, Diag. 2) about one-third of the origi- 

 nal macromere. Then follows a second cleavage, at right angles to 

 the first, cutting the larger segment into nearly equal parts i^x and 

 xy, Diag. 3 ) . The posterior macromere is now represented by three 

 sub-equal segments : one of these, x' , which may be called the 

 neuro-nephroblast ("primary neuroblast," in my first paper), lies 

 at the "posterior edge of the blastodisc, more on the right than the 

 left side. The second and third, representing the mesoblasts, are 

 also asymmetrically placed, one (;f) occupying a central posi- 

 tion at the lower pole of the egg (Diag. 3), the other {^x y) 

 lying behind it, and to the left of the neuro-nephroblast (.r'). 

 Neither in the general appearance nor in the relative positions of 

 the two mesoblasts is there anything indicative of their homo- 

 typical character. They appear to be vitelline spheres of the 

 same nature as the three macromeres, a, b, and c, and have 

 always been so regarded by earlier observers. In the course of 

 this period a shifting of position among the cleavage-spheres 

 takes place, which brings the mesoblasts into harmony with the 

 bilateral symmetry of the egg. The three macromeres lengthen 

 backward, slowly flowing over the mesoblasts and more or less 

 completely enveloping them, so that one or both of them may 

 entirely disappear from view, or, at least, become so obscure in 

 outline as not to be easily recognized. In its backward elonga- 

 tion the anterior macromere c takes up a median ventral posi- 

 tion between a and b, and usually carries the mesoblast x 

 towards the hind end at the same time that it incloses it. 



