OF OECAFTHUS AND TELEAS. 



210 



splanchnic mesoblast. The protoplasm of the endodermic cells forms an irregular outline 

 on the inner surface of the layer. Here and there may be seen the large endodermic 

 nuclei which have come to the surface of the yolk mass to fuse with the inner wall of the 

 mesenteron and to furnish by subsequent division the nuclei of other cells. The splanch- 

 nic mesoblast has been separated into two parts, a muscular layer in contact with the endo- 

 denn, and an epithelial layer of exceedingly thin flattened cells continuous with the lining 

 of the body cavity. The cells of the epithelial layer are spindle-shaped in cross section 

 and occur at irregular intervals. (PI. 22, fig. 9.) 



The formation of the heart does not begin until after the revolution of the embryo. 

 It is first to be distinguished in the abdominal region about the time of the closure of the 

 dorsal ectoderm. With a magnifying power cf 125 diameters it appears (pi. 19, figs. 4, 

 5 ; pi. 22, fig. 1 ; pi. 23, fig. 5) as a delicate tube with here and there nuclei lying in contact 

 with its wall, but when more highly magnified the wall is seen to be double. In the for- 

 mation of the heart, the lateral plates of mesoderm grow upward around the mesenteron, 

 and as their edges approach the median dorsal line there is seen to be in each a tube. 

 These two plates coalesce in the median line and their tubes unite into one — the heart, or 

 dorsal vessel. 



The mesoblastic plates are not divided in this region into splanchnic and somatic 

 layers until after the formation of the heart. 1 (PI. 22, figs. 10, 11, 12, and 14; pi. 21, 

 figs. 32, 33, 34, 35, 3G, and 41.) In cross sections of the cmbryoin the stage represented 

 in pi. 22, fig. 1, the origin and formation of the heart may be traced satisfactorily, and, as 

 has been shown above, it does not differ from the formation of the heart in Lumbricus as 

 described and figured by Kowalevski (Ice. cit. p. 25, pi. 7, fig. 23). In this stage the 

 heart is a V-shaped organ, with the stem of the V directed backwards, but as the limbs of 

 the V approach each other they fuse and in this manner form the single tube of the dorsal 

 vessel. The formation of the permanent vessel keeps pace with the 

 growth of the dorsal ectoderm from behind forwards. 2 The histological 

 elements composing the heart in Oecanthus deserve especial men- 

 tion. The primitive tube of each mesodermic plate is formed by a row 

 of cells, each one of which becomes (£ shaped, and then by the fusion 

 of the lips of the (£ it acquires a (^ j) shape. (PI. 21, figs. 33 and 36.) 

 These are the muscle cells of the wall of the heart. The nucleus lies 

 in the outer part of each cell. In some sections the mesodermic plates 

 appear to be composed at their upper edges of amoeboid cells, so that 

 in some parts the heart does not have any structural connection with 

 other portions of the body at this stage. In such a section the wall 

 may be destitute of nuclei, and the heart then appears as a 

 single-walled, round, oval or D-shaped vessel. Its cavity, before and 

 after the fusion of the two parts, is filled with the same coagulable fluid 

 that fills the body cavity. Mesodermic cells, such as compose 

 the wall of the heart, are found free both within and without the 

 vascular cavity at this stage. It is probable, however, that all these 



an. 



Fifr. 27. 



Diagram of the half-formed 



heart, an. its posterior end. 



1 Compare Butschli (11). the heart is formed first in the head region and extends into 



2 Compare the account by Claus (13) of Brancliipus where the abdomen at a later period. 



32 



MEMOIRS HOST. SOC. NAT. HIST. VOL. III. 



