No. 2.] THE EMBRYOLOGY OF LIMULUS. 209 



mental lines. The tissue from which they arise is the bound- 

 aries of the somites which extend inward toward the median 

 line and which by their encroachment into the yolk outline 

 the liver lobes. The early history of this portion is traced 

 in connection with the alimentary canal. In the abdominal 

 region the differentiation of the muscles of the gill appendages 

 is accomplished in the same way, and it is interesting to note 

 (see Fig. 79) that the anterior wall of the somite develops 

 the extensor and the posterior the flexor of the corresponding 

 appendage. 



Entosternite. — Passing between the alimentary canal and 

 the nervous system in the cephalothoracic region, and serving 

 at the same time to connect the pedal muscles of the right 

 and left sides is a layer of tissue which serves as a tendon, 

 or rather as a series 'of tendons, and which by its later chohdri- 

 fication* or chitinization (vide Lankester, '84, p. 133) gives rise 

 to the entosternite. It is to be noticed that in its develop- 

 ment the entosternite (Figs. 74, 84, Z6, '$,'j es) is always fibrous 

 and it arises from the fibrous tissue of the region. The other 

 "cartilages" occurring as axial tendons in the gills and oper- 

 culum (Fig. 79) present in their early stages a distinctly chon- 

 droid appearance. 



Organs of Circulation. — The early history of the central 

 circulatory organ — the heart — of Limulus was outlined in 

 my paper of 1885. Later, Kishinouye has added to the ac- 

 count given there and has corrected some points in my de- 

 scription. So this early history need not be detailed here. 

 The heart arises as a result of the extension of the mesoderm 

 over the yolk towards the dorsal median line. Its walls are 

 formed by the edges of the advancing tissue, and, according 

 to Kishinouye, as the walls of the tube thus formed are inter- 

 rupted in the intersomitic region, a series of segmentally 

 arranged openings into the cavity — the ostia — are produced. 

 The differentiation of the heart begins at first behind and 

 gradually extends forward. In its early formation the heart 

 of Limulus affords support to the theory of Biitschli ('82) in 

 regard to the relations of the circulatory system to the seg- 

 mentation cavity. 



