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', and although the subsequent cleavages grow more 
and more irregular, bilaterality is sometimes strongly expressed even in the later 
stages (cf. М. 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 [ 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 П (cf. his figs. 13-18, 33, 34, 36, 37-39, 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 іп Amphioxus to 
determine accurately 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?, В?, C?, D?) 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. Неге 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- 
! In the newt, Diemyctylus, Jordan (1893) found that the first cleavage is perpendicular to the 
median plane. 
