DEVELOPMENT. 197 



Owing to the rapid concrescence of the more posterior lateral plates, the yolk 

 territory behind the tail end of the embryo is covered at an early period. Hence 

 further apical growth must take place in a vertical direction, or in such a manner 

 as to raise the apex of the tail off the surface of the egg. (Fig. 157, D.} Under 

 these new conditions, the formation of both neural and haemal surfaces takes 

 place at very nearly the same time, and under similar conditions. The heart 

 does not extend into this region of the trunk. 



It is thus seen that there are three natural divisions of the haemal surface, 

 where the physical conditions are, necessarily, fundamentally different; namely, 

 in the cephalothoracic, in the abdominal, and in the caudal. The factors that 

 produce or control these conditions are the relations that exist between the rate of 

 apical and bilateral growth and the volume of the yolk sphere over which this 

 growth is obliged to take place. (Figs. 17, 23, 34.) 



Other Arachnids. In the scorpion and in spiders (Epeira), the heart develops 

 in essentially the same manner as in Limulus. (Figs. 15, 16, 17, 20, 22.) 



The details of the process of heart formation, as seen in sections, especially 

 the stages immediately preceding, and during the concrescence of the cardio- 

 meres, have not been worked out. They should receive more a careful study than 

 we have been able to give them. 



Comparison of Vertebrate and Arachnid Heart. A study of the early 

 stages in the development of the heart in arachnids and primitive vertebrates 

 shows that in both classes we are dealing with different phases of the same process. 

 In both classes, the heart is formed from the peripheral ends of lateral plates 

 belonging to a variable number of branchial metameres. (Fig. 32-33.) These 

 ends grow in an anterior haemal direction and concresce in the median haemal 

 surface, behind the anterior end of the forebrain and the cephalic navel, or 

 neostoma. 



In the arachnids, the heart is composed of a single layer of loose, striated 

 anastomosing muscle cells covered by a fibrous membrane. (Fig. 2.) The 

 heart tube is enclosed in a distinct pericardial chamber, the pericardial walls and 

 the walls of the heart being continuous at the anterior and the posterior ends, 

 and at the points where the aortic trunks arise. (Fig. 118.) The heart is also 

 attached to the pericardial walls throughout the entire length of the neural and 

 haemal surfaces by numerous connective tissue fibers and muscular strands. 



The pericardium on the neural side forms a tough fibrous membrane of con- 

 siderable thickness, but it does not appear to contain muscular bands. 



In the vertebrates, we may recognize the same kind of growth from the same 

 region to the same region. In its earliest condition, the vertebrate heart is an 

 axial cord composed of a syncytial meshwork with irregular, interstitial spaces. 

 (S. Mollier.) This is the so-called mesenchematous stage of the amphibian and 

 selachian heart. At a later period, the primary cord is metamorphosed into a 

 thin-walled tube, the endocardial tube, and a muscular layer, or myocardium, 

 forms around it. The two layers are separated by an extraordinarily wide space, 



