788 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [10] 



hepatic structures to the stomach in the adult, which is very apparent 

 when cross-sections are examined. While there seems to be only two 

 hepatic diverticula in tlie embryo, from the alimentary canal it is evi- 

 dent that the liver in the adult opens into the gastric cavity by way 

 of four principal ducts, one pair being more anterior in position than 

 the other J this may be a result of the transverse division of the primi- 

 tive ducts, just as the lobules of the liver are multiplied by the up- 

 growth of folds on the walls of pre-existing follicles, by which the 

 latter are again and again subdivided and multiplied in the course of 

 further growth. The hepatic tissues are most extensively developed 

 below and at the sides of the stomach in the adult, sparingly at the 

 upper part of its sides, and are altogether wanting immediately above 

 it along the median .line. Together with its increase of size the number 

 of its follicles increases very greatly so that there may be thousands in 

 the adult, whereas there were at first but two in the embr^'^o. 



Cross-sections of the soft parts of the young spat show the hepatic 

 follicles proportionally larger than in the adult, but far less numerous, 

 there being at most not over a few dozens present in spat of the size 

 shown in Fig. 2. 



The connective tissue also, which forms so large a proportion of the 

 soft i)arts of the adult, is very spariogly developed in the spat, of the 

 size here figured. In the young larva the connective tissue appears to 

 be represented only by a few multipolar cells and the cells which enter 

 into the formation of the anterior adductor and retractor muscles 

 of the velum. In the spat there does not appear to be any connective 

 tissue or mesoblast between the liver follicles (hypoblast) and the mantle 

 (epiblast), which forms the integument or body walls of this stage. The 

 development of connective tissue in such quantity as is found in the 

 adult therefore occurs during the time intervening between the develop- 

 ment of the earliest condition of the spat and the stage of growth 

 reached within the next following twelve months. In the course of its 

 development the connective tissue remains in part spongy, aud has a 

 lacunar structure in some parts, but in certain parts of the body-mass 

 its component cells enter directlj^ into the formation of the walls of the 

 principal vessels which are devoid of an endothelial lining. As the 

 stratum of connective tissue increases in thickness, the organs of epi- 

 blastic origin, the mantle and gills, and those of hypoblastic origin, the 

 alimentary canal and its appendages, are more widely separated from 

 each other by it. 



The vascular system and heart both originate from the connective 

 tissue or mesoblast. In the youngest stages of the larva there is no 

 heart developed, and inasmuch as the walls of the vessels traversing 

 the body-mass in the adult are formed of connective tissues alone, there 

 can, of course, be no vessels developed in the larva, where the connective 

 tissue is still practically undeveloped as a discrete layer. The colorless 



