nientai y tract. In later stages the fins become highly vascular and doubt- 

 less serve both for purposes of aeration and food absorption. 



IS. There is present in the entodermic pole of the developing egg a 

 body the like of which has not been observed in any other e?g It con- 

 sists of a mass of protoplasm imbedded in the yolk. It is dissolved near the 

 time of the closing of the blastopore. v Mr. J. W. Hubbard, one of my stu- 

 dents, has connected its history with that of the yolk nucleus which is 

 a conspicuous structure in the ovaries of adult fishes in egg from 20 y> up 

 to maturity. It is a general extrusion from the nucleus of the young 

 ovum and probably represents the histogehetic or somatic portion of the 

 nucleus and this in part at least corresponds to the macronucleus of ciliate 

 infusoria. 



19. Before segmentation begins the whole of the germ is separated 

 from thedeutoplasm. The first cleavage plane extends entirely through 

 the germ to the yolk before the second cleavage begins. 



20. A segmentation cavity is not formed during segmentation but 

 appears later by a separation of the ectoderm and entoderm. 



21. The third cleavage plane is not parallel with the first as is usual 

 in fishes, but is semi-equatorial. This has nothing to do with the hori- 

 zontal cleavage claimed to have been seen by Hoffman and by Brook. It 

 is taken to be a pseudoreversion to primitive methods oi segmentation 

 with the reservation that this condition is not perfectly homologous with 

 the third segmentation of the frog or Branchiostoma, and would not be had 

 the yolk entirely disappeared. 



22. The periblast is formed from a few of the marginal cells. Like the 

 yolk it is a waning structure. Only about 12 cells are ever formed. They 

 take no part whatever in the formation of the embryo. All of them per- 

 sist as long as a trace of the yolk is left. It, with the final part of the 

 yolk, is absorbed by the blood of the sinus venosus. The liver has noth- 

 ing to do with its final absorption as Wilson has claimed but simply me- 

 chanically encloses the nuclei above and behind. 



23. During an early stage of segmentation some of the marginal cells 

 of the blastoderm creep over the yolk till they nearly if not entirely 

 cover it. 



24. Before gastrulation the yolk sinks into the mass of the blastoderm 

 the cells of which re-arrange themselves about it and nearly enclose it. 



25. The uastrula is finally formed by a process of delamination of en- 



