776 PROFESSOR W. C. MSINTOSH AND MR E. E. PRINCE ON 
sections, Pl. XI. figs. 2, 3), indicating that the epithelioid layer is not formed in some 
Teleosteans simultaneously with the formation of the cardiac tube, and favouring the 
view that the heart becomes tubular by dehiscence of its median cells, or, as LEREBOULLET 
says, the linear cavity is formed partly by separation of cells and partly by absorption 
(No. 93, p. 551).* It seems probable that in different Teleosteans this organ has a 
different structure primarily, and certainly at later stages the circulatory system diverges 
in various species. Thus in the Gadoids, Pleuronectids, Trigla, and other pelagic forms, 
no yolk-circulation is ever developed, whereas in most demersal forms a circulation upon 
the surface of the yolk is a very striking feature, and may be said to a certain extent to 
precede the heart’s aetion ; for Truman found in Esox that blood-corpuscles were formed 
in patches in the cortex of the yolk, constituting the “ islands of blood-corpuscles” which 
Genscn has described (No. 56), and that before the heart pulsates, blood actually moves 
towards that organ, At the eighty-sixth hour TrumaN saw these moving corpuscles reach 
the heart, but it was ten or twelve hours later before the organ exhibited any motion, and 
even then no corpuscles passed into its cavity (No. 154, p. 191); so that the pulsations 
are independent of any stimulus given by the presence of blood-corpuscles within its 
chambers. Muscular twitchings, again, are often observed in the heart of the gurnard 
before the proper pulsations begin. We have already seen that the cardiac chamber is 
enlarged by the raising of the head of the embryo, and LEREBoULLET noticed that as this 
took place in Perca the heart becomes detached from the head, its anterior end following 
the retreat of the yolk, sinking slowly, while the hind end remains attached under the 
embryo. While yet a simple tube, the heart is contractile, the early pulsations, which 
commence usually one or two days after the heart is formed, being one of the most note- 
worthy features in the developing embryo, though no hemal fluid can be made out.t At 
first the pulsations are very slow and intermittent, the intervals between the contractions 
being irregular. In an embryo, four days after fertilisation, the beats are more rapid and 
regular, averaging 48 pulsations per minute, while the rate at times is greatly increased. 
Thus Dr Truman found in sow, soon after the heart began to beat (at the ninety- 
ninth hour), they reached 104 per minute (No. 154, p. 193), but the conditions 
must have been abnormal. The rate noted by LerEBouLLer in Perca, viz., 40, 50, or 
60 times per minute, is normal (No. 93, p. 451). In a ling of the second day (Pl. XIII. 
fig. 4) the pulsations were observed to have reached the rate of 80 beats per minute. 
The endothelial lining of the heart appears as a single delicate layer of cells, very much 
flattened and loosely suspended in the cardiac chamber, apparently derived from the 
myocardium or thick contractile layer. OXFLLACHER regards it as developed in the trout 
from the hypoblast beneath, and his figures on Taf. iv. (No. 114) are very clear; but 
no such continuity of the endocardium with the limiting hypoblast below is shown in 
* In certain insects Parren has found that two mesodermic plates by a median longitudinal fusion form a solid 
cord (Phryganida), while in others (Blatta) it is hollow from the time of its formation, and the mesodermic folds pulsate 
long before they unite to form the heart (Parren, “ Develop. of Phryganids,” Quart. Jour. Micr. Sci., vol. xxiv., 1884, 
pp. 587, 597). 
+ Trawling Report, 1884. 
