544 EEPORT OF COMMISSIONER OF FISH AND FISHERIES. [90J 



nuclei imbedded iu it, and in sections strands of its substance pass in- 

 wards to be insinuated between tlie coarse yelk masses in the interior, 

 which are composed of a different kind of granular protoplasm. The 

 The free nuclei are most abundant in the dorsal and anterior portion of 

 the yelk-hypoblast. No vessels or traces of any are ever found travers- 

 ing it, and with the approach of the later stages of development it is 

 not clear that the heart maintains its wide communication with the seg- 

 mentation or body cavity as observed at an earlier stage. The yelk, as 

 absorption proceetls and diminution of it« bulk results, assumes a fusi- 

 form shape or becomes somewhat like an oat-grain in form. All this 

 while, however, its anterior end continues to lie close to the heart and 

 may even be drawn out into a conical process, directed towards the venous 

 sinus. This conical process consists almost entirely of the yelk-hypoblast 

 or outer rind of the yelk proper, which does not disappear with the col- 

 lapse of the yelk, but is kept of about the same thickness until the whole 

 of it with its contained granular protoplasm is absorbed. In this col- 

 lapse we may also note another point of interest; it is that the yelk 

 diminishes behind and below so that its anterior ecd maintains its close 

 relation to the heart while the posterior end, as it recedes towards the 

 head, uncovers more and more of the under side of the liver behind and 

 above it. 



The mode in which the yelk hypoblast is continually kept of the same 

 thickness is very remarkable. While its substance is being removed 

 externally by the gemmation of blood-cells from its surface into the 

 segmentation cavity, as in Alosa, or into the vitelline vessels in Sahno, 

 its thickness is maintained by the apposition of material to its under 

 or inner surface from the underlying yelk, the internal ^^anular matter 

 of which is slowly transformed into the clearer and more homogeneous 

 plasma of the yelk-hypoblast proper. This transformation goes on 

 until the whole internal yelk mass is thus made transferable to the 

 nascent organism of the young fish, by means of the blood-cell gemma- 

 tion already spoken of. Yelk absorption is therefore a physiological pro- 

 cess of the most far-reachiug significance. The yelk itself may be com- 

 pared to the endosperm of a large seed in which tue stored proteinaceous 

 matters are slowly broken down by the agency of an organic ferment 

 and rendered soluble and diffusible through the cellulose walls of the 

 component cells of the infant plant. The analogy does not stop here, 

 however. If we look deeper, it is not improbable that we may hit upon 

 the ti'ue significance of another set of phenomena which have not, as far 

 as I am aware, been viewed in the light in which we propose to view 

 them in a succeeding chapter. 



The absorption of the yelk of the cod embryo is evidently siaiilar to 

 that of the shad. In Fig. 49 a yelk canal yo passes forward to the heart 

 from what is left of the yelk d. This canal is evidently similar to the 

 arrangement seen in Alosa, where there is an anterior conical process 



