190 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
tubules are still present, but the blood spaces are larger, and are 
much invaded and subdivided by mesenchyma, which is accom¬ 
panied by pigment cells. It thus appears that the pronephros per¬ 
sists probably through the entire larval period of the lamprey, and 
that the growth of the blood spaces in size exceeds that of the 
tubules, so that the pronephros becomes more and more like a mere 
blood sack, thus reaching a morphological character which has, so 
far as I know, no parallel in any other class of Vertebrates. 
Whether or not a similar condition exists in Myxinoids is still to be 
determined. 
Nothing can be said concerning the pronepliric circulation of 
Elasmobranchs, since the organ is rudimentary, and even during 
early embryonic life is represented by a transitory anlage only. 
In all the remaining fishes (teleostomes) the organ is well devel¬ 
oped, at least in embryonic and larval stages, and it sometimes 
persists in the adult. I have before pointed out, Minot, ’98.1, pp. 
271-272, the sinusoid circulation, as it may be observed in the 
teleosts, Ameiurus and Batrachus, and in the ganoid Amia. Of 
this last, Fig. 2 exhibits the appearance in an embryo of 10 mm. 
Fig. 2. Amia calva, embryo of 10 mm. Harvard Embryological Coll., No. 
19, section 262. Pronepliros in transverse section. Nell., notochord ; Pseudo., 
pseudolymphoid tissue ; Pig., pigment cells ; Si., blood sinusoid ; t., prone- 
phric tubule ; msth., mesotlielium ; Coe., coelom ; Ao., aorta. 
The organ may be said to consist of three parts, the pronepliric 
tubules, the considerable masses of pseudolymphoid tissue, very 
similar in appearance and staining to the corresponding tissue of 
teleosts, and of the large blood spaces, or sinusoids; these last are 
