530 EEPOKT OF C0M3IISSI0NER OF FISH AND FISHERIES. [76] 



dible beiug short and immobile. After the mandible has grown to be 

 of the same length as the upper it first begins to show signs of mobility, 

 though it is not untU some days after incubation that the jaws of the 

 embryo begins to mote, and then mostly rhythmically in respiration, 

 the water beiug sucked iu through the mouth and passed through the 

 gills, the same as in the adult. Only after the young fish have the jaws 

 distinctly developed, as in Fig. 49, do w^e begin to note that there are 

 voluntary snapping movements of the mandible manifested. 



At the time of the birth of the young cod there is no circulation of the 

 blood; there are no blood vessels, in fact; which accounts for the non- 

 functional development of the branchial apparatus at this period. The 

 function of respiration at the time of exclusion and for some time there- 

 after, as during development within the egg, is apparently performed by 

 the skin, which presents a large amount of surface, as may be seen in 

 Fig. 40. Not only is this true, but the skin itself in the living embryo 

 of this stage is lifted off perceptibly from the underlying structures, as 

 shown in Figs. 42 and 43. This subdermal space, filled with fluid, prob- 

 ably a serum, is of the nature of the serous space around the yelk, and 

 doubtless has a respiratory in addition to an assimilative function. The 

 existence and office of such spaces in embryos have hardly received the 

 attention they merit; they probably represent the earliest and most 

 unspecialized contrivances for the transfer of pabulum in solution, in 

 the form of paraglobulins, fibrins^ or other plastic matters, from one 

 part to another of a nascent organism. 



The intestine during the later stages of development is gradually 

 separated from the notochord at its hinder extremity by the interposition 

 between the former and latter of more and more tissue, mostly of a 

 mesoblastic character, which can scarcely be accounted for except upon 

 the supposition that in each and every cell of the embryo there inheres 

 a power of growth dependent again upon the intussusceptive powers 

 of the cells themselves, by which they are enabled to appropriate soluble 

 plasma through their neighbors or by way of intercellular or the exten- 

 sive serous spaces already alluded to. The gradual evolution of the 

 embryo fish before there is the slightest evidence of a systemic circula- 

 tion forces the foregoing conclusion upon the student. He sees, for 

 example, a germinal disk, at the commencement of development, of a 

 determinate form and size, but it is not long before he begins to dis- 

 cover that additional material from the yelk has been added to the 

 embryo, the bulk of the embryo itself perceptibly surpassing in size the 

 original bulk of the disk from which it took its origin. This has been 

 accomplished, too, iu all cases, before there is a trace of circulation; in 

 fact, before even the heart has begun to pulsate. It is this gain iu 

 bulk of the embryonic structures above and beyond the original mass of 

 the germinal disk Avhich cannot be accounted for on any other hypothe- 

 sis, as pointed out by Kauber. The segmentation cavities of the ova of 

 various types accordingly acquire a profounder meaning than has 



