BULLETIN OF THE UNITED STATES FISH COMMISSION. 161 



and in consequence of this difference alone we should not be surprised 

 to find lateral fin-folds of considerable extent in the embryo of the latter 

 and of limited extent in the former. Viewed in this way, we may j)rove 

 too much for the doctrine of the origin of the paired fins from lateral 

 folds. The truth of the matter appears to be that we ought to quietly 

 wait ibr more facts before geueraliziug- with the data ol)tained from only 

 one group. 



SUMMARY OF ANATOMICAL RESULTS, BASED ON ALL OF THE SPECIES 



STUDIED BY THE AUTHOR. 



The following" are the more important anatomical and embryological 

 facts which have been ascertained; onlj' such as are essentially new to 

 science or which receive new or fuller interpretations are noticed : 



1. The segmentation cavity or blastoccel is developed with the growth 

 of the blastoderm so as to almost entirely surround and include the yelk, 

 and persists until late in embryonic life. 



2. The somatopleure (muscular layer) and splanchnoi)leure (peri- 

 toneal layer) grow down into the segmentation cavity on either side 

 between the epiblast and hypoblast {Oncorhynchus). 



3. The heart develops in an extension of the segmentation cavity 

 beneath the head. 



4. The blood is of hypoblastic origin. 



5. The germinal disk is formed by the aggregation at one pole of the 

 germinal matter covering the vitellus {Cyhium), or scattered through it 

 as a meshwork joined to the peripheral layer (Alosa). 



6. Segmentation in the early stages is more or less decidedly rhyth- 

 mical, with alternating periods of activity and longer i^eriods of rest. 



7. The egg-membrane is not always a zona radiata, sometimes having 

 no pore canals; a micropyle is always present. 



8. The egg-membranes of the ova of certain genera {GJiirostoma, 

 Belone, Scomberesox, Hemirhaniphus) are provided with filaments or 

 thread-like appendages externally, by which they become atta,ched to 

 fixed objects in the water while hatching. 



9. The egg- membrane may be absent, and replaced by a highly vascu- 

 lar ovarian folbcie, j^erforated by a follicular foramen in some viviijarous 

 forms {Zygonectes). 



10. The median uni^aired fins originate from a dorsal and ventral natatory 

 fold, which may be continuous {Cyhium, Gachis), or be discontinuous at 

 the very first {Syngnathus, Hippocampiis), or be discontinuous very early 

 [Zyyonectes). 



11. The pectoral is the first of the paired fins to be developed fi^om a 

 short lateral horizontal fold, the position of which varies in different 

 genera, appearing very far back at first in some, farther forward and 

 nearer the branchial arches in others. 



12. The primitive cartilaginous coraco-scapular arch or shoulder- 

 Bull. U. S. F. C, 81^11 



