248 



roides, Lepidonotus and Podarke. The early cleavages vary from 

 nearly a pure radial to a very marked spiral form. In nearly every 

 case, however, the polar furrow is sufficiently well marked to be an 

 efficient guide to orientation, as is also the case in Podarke ^) and other 

 forms. In fact, different as the adult forms of Thalassema and Pod- 

 arke are, the early stages are so nearly identical that one set of il- 

 lustrations would almost serve for both. After the 64-cell stage, how- 

 ever, they begin to diverge and the end results, the trochophores, 

 are quite different. 



I will first describe the history of the pole-cells of the mesoblast 

 bands ("coelomesoblast" of Eisig, "entomesoblast" of Wilson), which 

 have the typical origin from 4 c?, the posterior member of the fourth 

 quartet. This quartet in Thalassema arises very early, in the cleavage, 

 at the transition from 32 to 36 cells. The four cells are all of the 

 same size and are budded off simultaneously (Fig. 1), As has hitherto 

 been found to be the case, the anterior (4&), and the right (4 c) and 

 left (4 a) lateral cells are "entomeres", while the posterior one alone 



/I ß 



Fig. 1*). Side view of 36 cell stage of Thalassema {A). View from lower 

 pole {B). The boundaries of the four quadrants, D, posterior; A and C, lateral; and 

 B anterior; are indicated by heavy lines. 



A, B, C, D, the four basal entoblasts or macromeres; 4, the fourth quartet of 

 "micromeres", 4d or 31 the ento- or ca?lomesoblast ; 4a — 4c entomeres; Sa — 3d, the 

 third quartet (ectomeres); Sa—3d, the second quartet (ectomeres) ; la— Id, the first 

 quartet (ectomeres). 



*) All the figures are made from camera outlines. The optical sections have been 

 confirmed by actual. 



1) Treadwell, a. L., 

 Morphol., Vol. 17, No. 3. 



The Cytogeny of Podarlse obscura. Journ. 



