154 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



tions appear in the walls of the brain tube, the cavity: inside becomes 

 divided into the so-called ventricles or cavities of the primitive cere- 

 bral vesicles. As development proceeds the cerebral vesicles rapidly 

 dilate in a lateral direction, especially the mid-brain mh^ in Avhich a snr- 

 prisingly spacious cavity is formed in some species, which answers to 

 the j)assage-way from the third to the fourth ventricles of the higher 

 forms. Between the fore-brain and mid-brain the pineal gland ^9w is 

 developed ; while the hypophysis cerebri or pituitary body de])ends from 

 the floor of the brain down between the trabecule cranii. The fore, 

 brain is at first not bifid or divided into hemisi^heres; its division occurs 

 comparatively late in embryonic life. The mid-brain is the most con- 

 spicuous portion of the encephalon or entire brain of the young fish, and 

 soon after hatching its lateral free lobes grow backwards and down- 

 wards somewhat at the sides, and more or less extensively cover the 

 cerebellum. 



At fourteen hours the embryos begin to show signs of the develop- 

 ment of pigment just below the superficial layers of the epiblast ; these 

 cells are at first scattered irregularly over the body of the embryo and 

 gradually grow darker; as they do this they also become irregular in 

 form and flattened, with a number of points running out from them, as 

 shown in Fig. 12. Later they tend to aggregate on certain parts of 

 the body, as shown in Fig. 13, where they form a band on the tail and 

 spots on the back ; as the embryo becomes still older a. band of them is 

 formed behind the ear. They are now still more irregular in form and 

 have evidently rearranged themselves very remarkably since the four- 

 teenth hour ; the rearrangement appears to be accomplished by their 

 migration towards definite points by means of an amoeboid movement 

 of their entire substance. When fully developed the nucleus becomes 

 very distinct, enveloped as it is in very dark protoplasm, and the pro- 

 longations of the latter look not unlike the pseudopods of those remark- 

 ably simple animals the Amcehce. 



By the eighteenth hour the oil sphere found embedded in the yelk of 

 the Spanish mackerel was observed to be enveloped in a mantle of cells 

 apparently of hypoblastic origin, which fastens it firmly to the wall of 

 the yelk sack below and opposite the embryo. Fig. 12. By the time 

 the young fish is ready to hatch, the covering of the oil sphere is found 

 to be more or less covered with pigment, which seems to have in part 

 developed in the cellular mantle, as indicated in Fig. 13. The fixation 

 of the bouyant oil sphere to the ventral wall of the yelk sack makes the 

 latter bouyant, so that when the young fish escapes from the egg-mem- 

 brane it is turned wrong side up, and is not until some time after hatch- 

 ing that it has the power to right itself and counteract the bonyancy of 

 the globe of oil. 



The heart of the young mackerel, like that of the cod, originates in a 

 mass of mesoblast cells, which are coarser in character than those in 

 the immediate neighborhood ; they appear to be budded off from the 



