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Part III. — Sixteenth Annual Report 



the yolk corpuscles are very large ; they are round or oval, show no visible 

 nucleus, and are almost twice as large as ordinary blood corpuscles ; they 

 show a fairly distinct boundary, though not so well defined as that of a 

 blood corpuscle. The yolk corpuscles in the egg of Species No. 3, 

 Rafifaele, are round or oval homogeneous refractive bodies ; some are pear- 

 shaped, with a fairly well drawn-out point, others are irregularly cubical. 



The corpuscles, when moving towards the heart (c, ligs. 11 and 12), 

 proceed either singly or in masses. Some move with a uniform slow 

 motion, while others move in a jerky motion. The latter movement is 

 especially seen as the corpuscles near the mouth of the heart ; they then, 

 with each pulsation of the heart, oscillate backwards and forwards in 

 a sort of pendulum motion. The length of the recoil is always less 

 than the anterior advance, and so the corpuscle ever gets nearer the heart, 

 the velocity of its oscillations steadily increasing until it arrives at and 

 enters the heart. They are then seen passing along the aorta to the tail, 

 and immediately thereafter appear in the caudal vein, from which they 

 pass into the yolk sac; they proceed over the ventral surface of the 

 yolk and re-enter the heart. Some of the corpuscles pass directly from 

 the posterior end of the yolk sac to the heart without stopping ; others 

 stick to the periblast and remain attached for longer or shorter intervals. 

 On the posterior surface of the yolk, where the caudal vein enters the 

 yolk sac, the periblast shows a well-marked furrow which has been worn 

 in it by the circulating fluid. That there is some sort of circulating fluid 

 present seems extremely probable, though its existence is not easily 

 demonstrated. The first corpuscles then are formed in the periblast and 

 pass into the heart with the circulating fluid. 



There is, however, a second method by which corpuscles are added to 

 the circulation, and that is by a process of budding. Soon after the heart 

 begins to pulsate processes are thrown out from the periblast covering the 

 anterior end of the yolk. These processes have various forms : some are 

 slender and pseudopodium-like, with finely drawn-out tips {p. and 

 figs. 2, 4, and 14), others have swollen apices (p., figs. 6 and 18). The 

 club-shaped process represented in fig. 18, yp., arises from the thick layer 

 of protoplasm surrounding the oil globule. In the swollen head of the 

 process there is a small refractive globule. This process sways to and 

 fro with the pulsations of the heart. Little blunt papillae are also present 

 in the periblast. From the tips of these processes little corpuscles are 

 budded off. The tip of a process slowly swells out into a little sphere, 

 and when the latter attains a size at which the attraction of the heart 

 overcomes the surface tension of the protoplasm it breaks off and moves 

 towards the heart. This process of budding is extremely slow. In figs. 

 5 and 5a a corpuscle (c.) is shown which has just been budded off from 

 the papilla {p.). These pseudopodium-like processes are seen in all of the 

 species mentioned above. The corpuscles which are budded off do not 

 appear to differ from the primary corpuscles seen on the periblast. They 

 are irregular in shape and form, and appear, in some cases at least, to 

 have nuclei. 



In the ova of Murainid(B (Raffaele) the yolk sac is \'ery large. The 

 yolk is almost all absorbed before the larva hatches. As the yolk is 

 absorbed the empty space in the yolk sac is filled up by an sesophageal 

 pouch"^ figs. 11, 12, and 14). This is a dilatation of the sesophagus 

 which projects into the yolk sac. At first small, it increases steadily in 

 size as the yolk decreases; just before hatching it fills almost the whole of 

 the yolk sac. This pouch thus lessens the area upon which the heart has 

 to act, and therefore increases the etficiency of the heart in the work of 



* Borsa stomacale (Raffaele). 



