198 Ohservatwns on the JDevelopnent Y''^^irl^^}^X^m!' 
evident at the lowermost part. In the round space it encloses is 
the lens, contained in a well-defined capsule ; the outline of both 
is perfectly circular. The mouth, or rather the situation of the future 
mouth, presents rows of rounded papillae (r), doubtless the source 
of the adhesive thread by which the fish suspends itself. The duct 
from the yelk-bag to the intestines is to be seen. The pectoral 
and dorsal fins are commencing to be developed. 
2^rd April, 10th day. — The primitive trunk seen coming off 
from the ventricle shows three divisions, two being right and left 
first arterial arches, the third the main branchial artery, which gives 
off branches to each side to form the other arterial arches. The 
rudiments of four branchial arches are now seen between the heart 
and the aortic circle. Between these arches are seen, in some of 
the fish, one, in others three arterial currents, the third being very 
small. These currents enter the aortic circle. 
Arterial currents are seen to pass behind the eye, ascending from 
the region of the mouth, and also in front of the eye, near the 
olfactory organ (t). The venous system appears the same as on the 
2"2nd. I observed that part of the venous return from the head, 
instead of coming from ^ to the heart, passed tailwards between 
the intestine and the yelk-bag, and entered the vein which joins the 
cardinal vein at q71. 
The olfactory organ (t) is situated in front of the eye, at the ex- 
tremity of the muzzle, and has an oval outline. The pectoral fin 
(])) gives faint vibratory movements. 
The heart is now becoming more enclosed in the growing 
branchial structures, and placed more in the middle line. The blood 
which issues from the cardinal vein on to the yelk-bag is not now 
confined to the median line, it spreads over the whole surface, run- 
ning in furrows or channels, having no distinct walls as a vein has. 
It is streaming all over in numberless paths, all tending to the 
heart. The object of this is to expose the blood more completely 
to the action of the water, whereby a respiratory process is effected. 
The blood, depreciated by its course through the tissues, is no longer 
sufficiently subjected to the action of the water, owing to the fish 
growing rapidly and becoming more dense, a greater distance being 
interposed between the blood-vessels and the water. To remedy 
this, the great vein distributes its contents over the wide surface of 
the yelk-bag, where the blood is freely exposed to the surrounding 
element. In fact, this distribution of the blood in a large number 
of channels over the yelk~bag is a temporary breathing apparatus, 
which is designed to effect respiratory changes until the branchia3 
or gills are ready to perform that office. (Fig. 26 is an exceedingly 
rough diagram, showing the course of the blood from h, the entrance 
of the vein on to the yelk-bag, to a, the heart.) The ramifications 
of the vein are very numerous, and the blood-corpuscles hurrying 
