188 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
dered it easy to obtain much better sections than Muller could have 
had. These better sections show many details, not yet described, 
and especially render clear the circulation of the organ. It has, 
therefore, seemed to me desirable to report my own observations, 
especially as regards the pronephric circulation. The accompany¬ 
ing Figure 1, is from an 
Ammocoetes of 37 mm. 
The median plane of the 
embryo is to the left of 
the figure, which shows 
the natural inclination of 
the organ downwards 
and outwards; about 
one third of the actual 
section of the prone¬ 
phros was drawn. The 
pronephros, at this stage, 
may be described as a 
huge blood chamber in 
which the pronephric 
tubules are suspended. 
The tubules are loosely 
connected together by 
thin strands of tissue, in 
which pigment cells ap¬ 
pear ; pigment cells are 
also numerous over the 
surface of the organ, but 
their abundance is exac;- 
gerated in the figure, 
owing to the lateral 
wall being cut obliquely. 
The enormous size of 
the blood space is veiy 
striking, and the com- 
Fig. 1. Petromyzon jluviatilis, 37.0 mm. 
Transverse Series, Harvard Embryological Col¬ 
lection, No. 249, section 69G. Ventral part of 
the pronephros. Endc., endothelium; Pig., 
pigment; SL, blood sinusoid ; t., t., pronephric 
tubules; Nst., nephrostome; msth., mesothe- 
lium. 
munication between the 
parts of the blood space is so free that it can hardly be described 
as a series of anastomosing sinusoids. In other parts of this prone¬ 
phros, and in the opposite pronephros of the same section, the 
tubules are much less widely separated, so that the type is more 
