BULLETIN OF THE UNITED STATES FISH COMMISSION. 25 



ou either side of the embryo and in front of the head, which answers to 

 it. It is, however, greatly obscured afterwards, if not obliterated at a 

 comparatively early period, by the remarkable way in which the blood 

 vascular system of the embryo is formed. 



On the fourth or fifth day after impregnation, the primary divisions 

 of the brain are marked off, one of the most striking characters being 

 the extraordinary dimensions of the cerebral vesicles, the walls of the 

 brain cavity being thinner proportionately than I have ever found them 

 in other forms. The optic cups also differ in their structure from those 

 found in other fishes, in that there is a great space between the floor 

 of the cup and the lens, the origin of which from an indui^lication of the 

 epiblast may be very readily traced. Immediately behind the auditory 

 vesicles, and shortly after their invagination, the rudiments of the breast 

 fins appear as a pair of longitudinal folds. These therefore origmate 

 closer to the branchial arches than those of any other species studied by 

 me. As thej' often are found to originate on either side of the embryo 

 above the posterior end of the yelk-sack, and near a vertical from the 

 point where the vent appears. This latter is their mode of development 

 in the cases of the young of the moon-fish [ParepMppus) and the Spanish 

 mackerel {Gyhium maculatum). In the stickleback, however, there is 

 an extraordinary acceleration in the develoj^ment of the breast fin, so 

 much so that by the time the young fish leaves the egg, the breast 

 fins are as greatly developed as in a mackerel four days old. The pig- 

 mentation of the young stickleback is also accomplished at a very 

 early period, so rapidly, indeed, that it soon becomes impossible to see 

 the viscera through the mantle of pigment cells. There is another com- 

 plication which needs mentioning here, and that is the fact that a second 

 kind of brown i)igment cell, much larger than the black ones api^ears 

 on the skin before the young slips out of the egg. These brown cells 

 blotch the embryo on the sides and back somewhat symmetrically, and 

 foreshadow the style of pigmentation of the adult. 



The heart appears about the fourth day as a heap of mesoblast cells 

 just l>elow and behind the head, and is at first a simple spherical sinus. 

 It does not begin to contract vigorously until the seventh day, when its 

 pulsations are nearly if not quite 100 per minute. Its venous end rap- 

 idly elongates until it extends fully the diameter of the body beyond 

 the right side of the embryo, a large pericardial space being developed 

 below the head at this point for its lodgment, which space dips down 

 deep into the amber-colored vitellus. It keeps contracting from this 

 time onwards, but there are as yet no blood corpuscles. A large space 

 now appears on the right side of the embryo and underneath the latter. 

 This we may consider a venous sinus or channel of indefinite outline. 

 The floor of this space is, as far as I have been able to convince myself, 

 formed of the hypoblast from which knobbed cells project uj) wards, 

 which appear to be budding off portions of themselves which will be- 

 come blood corpuscles. Now follow amoeboid contractions of the yelk 



