212 



KINGSLE Y. L ^'OL. VIII. 



of the arterial wall. In other words the artery is not morpho- 

 logically inside the artery. 



In Stage K, as shown by the reconstruction Fig. TJ, this 

 process has been completed at the anterior end of the ventral 

 cord. Below, this figure shows the neural artery {av>j repre- 

 sented as filled with a solid mass, while the omission of the 

 nerve cord leaves a central cavity from which proceed the 

 openings for the nerves on the sides. In front (to the right) 

 below, the forwardly directed process shows the nature of the 

 outgrowths by which the ventral artery is extending itself 

 beneath the brain. 



In Stage L the dorsal portion of the neural artery has reached 

 the abdomen, while in the somite of the fifth appendage the 

 conditions are advanced as far as shown in Fig. 74. In the 

 oldest embryos I have sectioned the arteries surrounding the 

 nerves have extended themselves into the appendages (Fig. 89). 



I have not attempted to follow the development of the blood 

 sinuses, etc. They occur most abundantly in the abdominal 

 region (Figs. 65, 68, 73, etc.) and that they are produced by a 

 splitting of the mesoderm is easily seen in the development of 

 the gills. The pericardial sinus belongs to the category of 

 these blood spaces, the coelom taking no part in its formation. 



Comparisons. — I have above referred to Kishinouye's account 

 of the formation of the heart in Limulus with which my recent 

 observations are in fair accord. In my first account I described 

 the heart as arising from a solid cord of cells but this was a 

 mistake. A re-examination of the slide showed that at the 

 stage described the heart was already formed and its cavity was 

 filled with blood corpuscles. 



In the Arachnida most observers have described the heart 

 as arising from the coalescing edges of the somites, meeting 

 in the dorsal median line, there being slight differences in de- 

 tails between the Scorpions (Kowalevsky and Schulgin, Laurie) 

 and the Araneina (Schimkewitsch, Locy, Morin, Kishinouye). 

 In the other groups, as far as I am aware, no detailed obser- 

 vations have been made. 



The pericardium of the spiders, according to Schimkewitsch 

 ('87) arises as a layer of mesoderm split off from the splanch- 



