78 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



symmetrical. The first cleavage always coincides with the median plane among 

 ascidians, and every subsequent cleavage is perfectly bilateral, one-half of the egg 

 being the mirrored image of the other. In the frog's egg the first cleavage usually 

 lies in the plane of symmetry 1 , and although the subsequent cleavages grow more 

 and more irregular, bilaterality is sometimes strongly expressed even in the later 

 stages [cf. M. Schultze, 1863; Rauber. 1882). 



In Amphioxus, if I correctly understand Wilson (1893, p. 600), the first cleavage 

 coincides with the median plane. In the subsequent cleavages, both Wilson and 

 Samassa (1898) have been unable to find the remarkably regular alternation of 

 meridional and latitudinal cleavages described by Hatschek. These cleavages are 

 extremely variable in form; among them Wilson recognizes three principal types, 

 one radial and two bilateral. After the 16-cell stage, however, almost all the eggs 

 become bilateral, whereas in the 8-cell stage three-fourths of them are radial. 

 Wilson suggests that variations from the bilateral type may occur among ascidians, 

 but I agree with Castle and Samassa that under normal conditions this is never the 

 case. In the 8-cell, 16-cell and 32-cell stages of the bilateral types there are many 

 striking resemblances to corresponding stages of the ascidian ; this applies particu- 

 larly to Wilson's bilateral type II (cf. his figs. 13-18, 33. 34, 36, 37-30, 41-43, and 

 Samassa' s figs. 2, 6, 7, 9). In these figures the form of the cleavage is so similar 

 and the position of the cells and even the direction of the spindles within the 

 cells so remarkably like what is found in the ascidians that the individual cleavage 

 cells can be correlated in these two animal classes. 



Too little is known of the cell-origin of the germ layers in At>iphio.\us to 

 determine accuratelv how close is the likeness to ascidians in this regard. 

 Wilson holds that the eight animal cells of the 16-cell stage are purely ectodermal 

 and that the "secondary macromeres " (A 2 , B 2 , C 2 , D 2 , ) which surround, and were 

 derived from, the four basal cells at the vegetal pole are of mixed character, 

 giving rise to both endoderm and ectoderm, and perhaps also mesoderm. He does 

 not give the evidence upon which this conclusion rests, but its similarity to the con- 

 ditions which exist in the ascidians should not be overlooked. Here also the 

 eight animal cells are purely ectodermal, while the " secondary macromeres." and 

 in fact, all the cells of the vegetal hemisphere in the 16-cell stage are of mixed 

 character, the four anterior ones containing endoderm and ectoderm (neural plate 

 substance), and the four posterior ones, endoderm and mesoderm. Wilson expressly 

 states that he uses the terms macromere and micromere "solely for the sake of con- 

 venience," and he concludes that the cleavage is very unlike that of annelids ; 

 Samassa also emphasizes this same conclusion. 



We may conclude, then, that there are certain fundamental resemblances be- 

 tween Amphioxus and ascidians in the matter of cleavage and that the most notable 

 differences between them are found in the number of cells and the degree of their 

 differentiation at any given embryogenic stage ; in ascidians this number is rela- 



1 In the newt, Diemyctylus, Jordan (1893) found that the first cleavage is perpendicular to the 

 median plane. 



