504 EEPORT OF COMMISSIONER OF FISH AND FISHERIES. [50] 



marked extent. Doubtless, the iiifiindibuluiu is also partially developed 

 as a dowugrowth of the floor of the braiu, but this need not involve the 

 whole encephalon in a downward flexure. During the earliest stages 

 of the brain development here described, the whole brain region is but- 

 little wider than that of the body ; with its more advanced development, 

 however, the cephalic end of the embryo widens, in consequence mainly 

 of the rapid growth of the mid-brain in a lateral direction. If a section 

 of an embryo's head is prepared of the age of Fig. 30, cutting through 

 the region of the mid-brain, the mid-brain cavity will be shown as a 

 cruciform oi)ening, the lateral portions passing into the hollow lateral 

 lobes, mh Fig. 30, which are pushed out towards the eyes. AY hen, how- 

 ever, sections of much later stages are prepared this arrangement dis- 

 appears ; the optic thalami have acquired greater development; in fact, 

 the whole ventral portion of the mid-brain has augmented in volume, 

 and the origins of the lateral lobes seem to have been elevated while 

 the lobes themselves curve down over the underlying brain substance, 

 abutting laterally against the eyes and behind against the cerebellum. 

 The lateral lobe of the right side of the mid-brain is shown at U in Fig. 

 28. 



Behind the infundibulum, shown in Figs. 28, 29, 30, and 32, the medulla 

 oblongata has a very thick floor, in just hatched embryos, while the 

 roof of the fourth ventricle, contained in it, is quite ^hin. The thick 

 floor of the medulla in embryos of Alosa, three days old, passes straight 

 forwards over the infundibulum to just above the optic thalamus. The 

 bundles of commissural fibers connecting the various parts of the brain, 

 especially those arising from the floor and traversing its substance in 

 various directions, I have not traced. This portion of the medulla is 

 shown in the optic section represented in Fig. 28, 



The pineal gland appears as a mesial outgrowth from the anterior 

 portion of the mid-brain, being fairly developed on the eve of hatching 

 or shortly before it, as indicated in Fig. 32, at pn, in a side view of an 

 embryo cod on the sixteenth day of incubation. This organ has recently 

 received a good deal of attention on account of the relation it has been 

 supposed to bear to the primitive mouth of the vertebrates. Goette has 

 described it as being a product of the point where the roof of the brain 

 remains longest attached to the external skin in amphibian embryos, 

 and he compares the pineal gland to the long-persisting pore which 

 leads into the neurula of the embryo of Amphioxiis. 



Some days after hatching I have taken the embryos of Alosa and pre- 

 pared longitudinal vertical sections of the head in order to discover 

 what might be the true relation of the hypophysis to the infundibulum. 

 Dohrn * has recently investigated this feature of the development of 

 Teleostean embryos and has arrived at the conclusion that the hypoph- 

 ysis is really formed from the hypoblast and not from an epiblastic 



* Studien zur Urgeschichte des Wirhelihierkorpera. Mitth. aus der zool. Station zu 

 2feapel, III, 1881. 



