^ j?u™aifocr^^^^^^^^ of the Ovum of the Pike. 191 
the vessel proceeding from it ; also the vertebrae, and the rounded 
tail.) (Fig. 12 shows, /, the ear-capsule ; ^, the membrane en- 
closing the heart; it also shows the union of the halves of the 
brain in front, and the folding of the mass to form the division 
between the lobes of the brain ; and the median sulcus or 
canal.) 5 p.m., 79 hours. — The yelk surface is occupied by round 
clear spaces in which blood-corpuscles can be seen. This was the 
first time I had seen any, and 1 had watched very closely for their 
ajjpearance. By means of the contractions in the yelk, formerly 
mentioned, the surface is thrown into a series of wavy folds. In 
the furrows between these Kttle ridges are numerous blood-globules, 
which are passing towards the prsecordial area. There are blood- 
globules on both right and left sides of the yelk. The praecordial 
area is of a clear white colour, and empty: no blood as yet has 
reached it ; and it is to be observed that the heart is motionless, — 
that the blood flows to the heart independently of the heart's action. 
The blood-corpuscles are colourless at present. The oscillation of 
the embryo and yelk is vigorous. (Fig. 13 shows the wavy folds 
which apparently were the means of the movement of the blood- 
corpuscle on to the heart.) 11.15 p.m., 85 J hours. — Blood-corpuscles 
have reached and entered the praecordial area. 11.30 p.m. — There is 
in the embryo a duct leading from the interior of the yelk into the 
digestive canal ; by this means nourishment is conveyed to the em- 
bryo, and it is probable that the yelk contractions are the agents in 
propelling the yelk into the canal. (Fig. 14 shows this duct.) 
18//^ April, 10 to 12 a.m., 96 to 98 hours.— For the first time I 
observed the heart to be beating, yet no blood-corpuscles v/ere pass- 
ing through it. So that it is evident that the contractions of the 
heart are independent of any stimulus given by the presence of 
blood-corpuscles within its chambers. The heart in the adult fish 
consists, not of four chambers, as in the mammalia, but only of 
two — first, an auricle to receive and collect blood ; immediately 
succeeding to this a ventricle to contract on the fluid contents and 
force them onwards. Succeeding to this is the dilatation of the 
efferent vessel known as the bulbus arteriosus. In the case before 
us the heart consisted of a bag-shaped organ, divided into anterior 
and posterior chambers, the former being the wider of the two, and 
communicating by a circular constricted portion with the ventricle. 
The posterior chamber consists of two broad flaps, connected by a 
thinner structure ; the flaps move to and from each other, and thus 
are capable of producing an intermittency of current by shutting 
off the supply of blood during the auricular contraction. It may 
be pointed out that this period is a most favourable one for investi- 
gating the nature of the movements of the heart. The structures 
are all nearly as transparent as glass, and there is no current of 
blood, so that the conditions are eminently favourable for observa- 
