2 1 o KINGSLE Y. [Vol. VIII. 



My present description begins with Stage H (Fig. 64). In 

 this the heart may be seen with walls in which no definite 

 arrangement of cells is visible and with two blood corpuscles 

 in its interior. It is connected in this section with the dorsal 

 ectoderm by a cord of cells, while on either side are two 

 cavities, a dorsal and a ventral. The ventral is clearly the 

 coelom of the somite, and its walls, somatoplure and splanch- 

 noplure, are perfectly distinct. The upper cavity, the peri- 

 cardium, is plainly lacunar, and is produced by a splitting of 

 the mesoderm, and at this early stage is limited distally by 

 the trabecular tissue so characteristic of the embryo of 

 Limulus. Farther forward (Fig. 66) the heart is larger, and 

 the cells of its walls are arranged in a single layer. 



In the next stage (I, Fig. 66) the somatopluric mesoderm 

 has given rise to the alary muscles which are best developed in 

 the anterior portion. In the abdominal region (Fig. 6^), the 

 heart is larger, but in all parts it as yet consists of the single 

 layer of cells which were found in the preceding stage. Fig. 

 (ij, which passes through the plane of the genital operculum, 

 shows on either side the posterior extension of the dorsal 

 coelom. A few sections further back (Fig. G'i) this cavity dis- 

 appears from the sections. In longitudinal section (Fig. 82) 

 the heart is seen to extend back to about the middle of the 

 abdomen and forward to the anterior end of the yolk, following 

 this down toward its junction with the stomodaeum. At its 

 anterior end the heart divides into two aortic arches (the 

 "crosses aortiques " of Alph. Milne Edwards, '73) which I 

 have called the sternal arteries. These arteries (Fig. 'jG) pass 

 down one on either side of the stomodaeum to dispose them- 

 selves at first as two tubes upon the upper surface of the 

 ventral nerve chain. I have not satisfied myself of the way in 

 which these sternal arteries arise but the observations which I 

 have made are not incompatible with the view that dorsally at 

 least they are interseptal. This point is however difficult to 

 settle on account of the numerous lacunae, which, as already 

 mentioned, early appear in the mesoderm and confuse the 

 observer. At this stage (I) no other arteries arise from the 

 heart. 



