240 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



SECTION 4. The Segmentation of Salpa Hexagona as Compared 



with tliat of Clavelina. 



Seeliger has published a minute and fully illustrated account of the 

 segmentation of the egg of clavelina, based upon observations upon living 

 eggs, and my own somewhat scanty series of stages in the segmentation 

 of Salpa hexagona shows that it agrees very closely with clavelina, at 

 least during the early stages. 



The first cleavage plane divides the eggs of clavelina, according to 

 Seeliger, into a right and a left half, and each of these then divides, in a 

 plane at right angles to the first, into two cells, one of which lies in the 

 anterior and the other in the posterior region of the body of the future 

 embryo. My Fig. 3, Plate X, shows one-half of an egg which has 

 divided into two cells on the middle plane, and the nuclear figures show 

 that each half is about to divide into an upper and a lower half, corre- 

 sponding to Seeliger's anterior and posterior cells. In clavelina each of 

 the four cells at first contains both ectodermal and endodermal plasma, 

 but the next plane of segmentation, which is at right angles to both the 

 first and second, results in the division of each of these cells into two, one 

 of them dorsal and ectodermal, and the other ventral and endodermal. 

 The eight-celled stage is bilaterally symmetrical with two ectodermal 

 cells and two endodermal cells on each "side of the middle line. In a 

 symmetrical view of the dorsal surface the four ectodermal cells are 

 visible ; the four endodermal cells are seen in ventral view, and in side 

 view the two ectodermal cells and the two endodermal cells of one side of 

 the body are seen. The study of the series of sections of the eight-celled 

 stage of Salpa hexagona shown in Plate IX, Figs. 1 to 10, will show that 

 it agrees exactly with clavelina. It is bilaterally symmetrical with four 

 cells on each side of the middle plane, and four also in the upper half 

 (anterior?) and four in the lower half (posterior?) ; four cells have their 

 broad ends proximal or towards the pole cell (ectodermal or dorsal ?) and 

 four are distal (endodermal ?). 



As shown by the nuclear figures in' Figs. 6 and 7, the planes of 

 segmentation which result in the sixteen-celled stage are parallel to the 

 second plane of segmentation, while in clavelina, according to Seeliger, 

 they are parallel to the first plane of segmentation. Up to the sixteen- 

 celled stage, however, the agreement between the two is exact. 



