152 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



parent cells become crowded upon each other, and now compose the 

 entire medullary substance of the uotochord, which is functionally the 

 backbone or axis of the embryo fish. This metamorj)hosis of the primi- 

 tive chorda cells begins about the twelfth hour in the embryo mackerel, 

 and is completed by the time of hatching, and, like in the herring as 

 described by Kupfifer, the caudal end of the uotochord is the last to un- 

 dergo the change. The uotochord for a considerable time after hatch- 

 ing does not become distinct at its caudal end from the cellular mass in 

 which it terminates. The membranous sheath in which the uotochord 

 is inclosed seems to be differentiated when the metamorphosis of the 

 l^rimitive chorda cells into the clear axis rod or uotochord has been 

 completed. I have not succeeded in demonstrating from whic"h one of 

 the primary embryonic layers of the mackerel the primitive chorda and 

 consequently the uotochord are derived ; the weight of the evidence 

 afforded by the researches of others would appear, however, to indicate 

 that it is split oft" from the lower edge of the keel or carina of the 

 medullary canal just over the hypoblast. 



The axis of the embryo is at first marked by a shallow groove which, 

 by the time the blastoderm is closed, is comi)letely obliterated, the last 

 portion of it to disappear being the caudal. The blastoderm is pushed 

 down before this groove, and when the latter closes dorsally and the me- 

 dullary canal, neural canal, or neurula is formed, as it has been vari- 

 ously called. Certain anterior ijortious of it become difterentiated in- 

 to the various i)arts of the brain. Primitively the brain of the mack- 

 erel is much compressed laterally, as shown in Fig. 8. At the extreme 

 anterior end a pair of lateral outgrowths, at first apx)arently solid, ap- 

 pear as the rudiments of the eyes, ojpv. The basal portions by which 

 they a^e in communication with the fore jjart of the brain become 

 Xjartiatiy the optic or second pair of nerves. With the more advanced 

 development of the embryo these outgrowths become hollow and cup- 

 like, the retina of the eye is develop^'''on their inner surfaces, while a 

 mesoblastic layer of pigment cells is developed on the outside to form 

 the choroid coat. The cup has a cleft in its lower margin which closes 

 later, and is known as the choroidal fissure. Covering the ojitic cups is 

 the embryonic epithelial stratum of cells ; from it an induplicatiou is 

 j)ushed into the cups, which is eventually constricted off from the parent 

 layer, and becomes differentiated into a highly refringent spherical 

 lens. Between the lens and the floor of the cup a space is formed very 

 early whi6h becomes the vitreous humor of the eye, and in front the 

 lens is again roofed over by a very thin concavo-convex hyaline mem- 

 brane, the cornea, likewise derived from the epidermis, between which 

 and the lens the aqueous humor is confined. The iris appears to be 

 developed from the extreme edges of the optic cuj) and becomes very 

 brilliantly i^igmented in a few days after the fish is hatched. The an- 

 terior part of the brain, from which the optic cups grow out, becomes 

 the cerebrum or fore brain, in x)art, also, the optic chiasma. The spinal 



