No. 2.] THE EMBRYOLOGY OF LIMULUS. 211 



In Stage K the conditions are much the same, the relations 

 of the anterior end of the heart and the sternal arteries being 

 shown in Fig. ']'], drawn from a wax reconstruction. 



In Stage L, the heart (Figs. 72, 73, 82) has nearly attained 

 its adult condition so far as segmentation into chambers is 

 concerned. In Fig. 73 — representing a horizontal section — 

 the anterior end of the heart is shown, enclosed in the 

 pericardial space and supported by the alary muscles. In 

 front, on either side are the roots of the sternal arteries but I 

 have not seen at this or any earlier stage the frontal arteries of 

 Milne Edwards. Fig. 72, taken at a lower level, shows the 

 section of the sternal arteries on either side of the narrow duct 

 connecting the proventriculus with the mesenteron. 



It is not until late Stage H that the sternal arteries reach 

 the nervous system. At first they extend themselves as two 

 separate tubes along the dorsal surface of the nervous cords 

 and extend backwards but a slight distance upon them forming 

 the rudiments of the neural artery. There is at this time no 

 trace of any tube beneath the nerves. It is especially inter- 

 esting that this condition which is transitory in Limulus should 

 be permanent in the Scorpion. 



In Stage I the neural artery extends back -behind the middle 

 of the cephalothorax but its termination is indistinct. In 

 somite VII (Fig. 6^) no traces of it are to be found*. The 

 partition between the two arteries still persists (Figs. 70-71) 

 but on either side the artery is extending itself around the 

 nervous system and appearing beneath it, thus giving rise to 

 the peculiar condition so well known as characteristic of the 

 adult horseshoe crab. This condition is brought about, at 

 least in part, by outgrowths from the dorsal tubes but whether 

 there be cavities formed independently beneath the cord which 

 are later taken into the neural artery I cannot say. Most of 

 it is accomplished by the downward growth and the wrapping 

 of these portions around the cord, there being as Milne Edwards 

 has suggested a soldering of the two edges of the vessel and a 

 subsequent resorption of the resulting lamella. As will be 

 seen, by this process of formation the nerve does not float freely 

 in the blood but is surrounded by a neurilemma which is part 



