776 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



sections, PI. 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 action ; 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 

 Gensch 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 haemal 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 Esox, 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 Lereboullet 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. Oellacher 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 Patten has found that two mesoderinic 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 (Patten, "Develop, of Phryganids," Quart. Jour. Micr. Sci., vol. xxiv., 1884, 

 pp. 587, 597). 



t Trawling Report, 1884. 



