DEVELOPMENT. 197 



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

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

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

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

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

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

 ;s not extend into this region of the trunk. 



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

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

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

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

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

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



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

 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 

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

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

 have been able to give them. 



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

 ,ges in the development of the heart in arachnids and primitive vertebrates 

 3ws that in both classes we are dealing with different phases of the same process, 

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

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

 is grow in an anterior haemal direction and concresce in the median hasnial 

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

 ostoma. 



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

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

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

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

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

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

 ;m.al surfaces by numerous connective tissue fibers and muscular strands. 



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

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



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

 ^ion to the same region. In its earHest condition, the vertebrate heart is an 

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

 . MoUier.) This is the so-called mesenchematous stage of the amphibian and 

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

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

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



