536 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [82] 



Size as the yelk is absorbed. This relation of parts is merely pointed 

 out hero in defense of the position which has been assnmed in regard 

 to the non-connection of the glomeruli with the body cavity, and will be 

 again alluded to in its proper place. 



22. — The development of the heart. 



The development of the heart of the young cod is typical of that pro- 

 cess in young osseous fishes generally, with this qualification only, that, 

 like the history of other portions of the body, the details of it differ con. 

 sidcTably in the embryos of different genera. The earliest evidence of 

 its presence is represented in Fig, 30, showing the under side of the head 

 of an embryo of fifteen days. Large cells have first arranged themselves 

 in a transverse band-like layer below the fore-gut; the latter is not repre- 

 sented in the figure. This layer is continuous with a stratum which is 

 continued forwards on either side of the fore-gut and mid-brain, and for- 

 wards in front of the eyes and below the fore-brain. It is mesoblastic, 

 and the only portion of it which has crept around to below the fore-gut 

 is the. transverse band in which the heart develops. Sections through 

 the heads of embryos in similar stages of other genera justify the fore- 

 going description. This mesoblastic stratum is also continuous with the 

 posterior pericardiac septum j?c, just behind the heart, while at the ex- 

 treme anterior end of the head it contributes to the formation of the 

 trabeculoB cranii and rostral plate or cartilage, and behind and above 

 the gut it shares in the development of the parachordal sheath. In the 

 extremely early stage of heart development shown in Fig. 30 the cavity 

 of the fntnre heart is a circular opening li in the plate of cells, from which 

 the organ is formed by growth in length, v It now appears to be wider 

 than deep, but it gradually elongates, and instead of its axis remaining 

 vertical to that of the long axis of the head, it grows forwaj*d more or 

 less horizontally, as seen in Fig. 29, where the head of a scventeen-days 

 embryo is viewed from below. Its anterior extremity now widens into 

 a funnel shape and begins to pulsate very slowly and irregularly — once 

 or twice in a minute, perhaps. Its interior, apparently 0])en end is not 

 free, however, as might be supposed from an inspection of the figure, but 

 is continued. into an exceedingly delicate, thin membrane, continuous 

 behind with the pericardiac septum. It will also be noticed that in Fig. 

 29 its front end is bent to the right side. This lateral bending is con- 

 tinued during the progress of developnjent, and finally when the einliryo 

 is hatched it opens backwards, as shown in Figs. 40, 42, and 46, and also 

 in 41, as viewed from the side, where the primitively anterior end is 

 shown at sv. What was the lower side of the cardiac plate in Fig. 30 

 has grown downwards, then forwards, then is bent to the right, as in Fig. 

 26, then still more in Fig. 42, till, as viewed from the side in Fig. 41, 

 the heart tube forms a looj) with what wns formerly the front end directed 

 backwards and upwards and joined to the pericardiac membrane. It 

 has now passed through the following stages : First, it has the form of 



