DEVELOPMENT OF THE IIEIMMNO. 561 



"Tin- young, wlii-ii they emerge from tin- cgj,% arc from one lift h to one-third of an inch in 

 length, and so extremely unlike tin- adult Herring tliat they may properly be termed 'larviw.' 

 They have enormous eyes and an exceedingly slender body, \\ iili a >elk l>ag protruding from its 

 fore part. Tlie skeleton is in a very rudimentary condition; there are no ventral tins; and, in-ii.nl 

 of separate dorsal, caudal, and anal tins, there i.s one continuous fin, extending from the head 

 al< MI ^ the back, round the tail, and then forward to the yelk-bag. The intestine is a simple tube, 

 ciliated internally ; there is no air-bladder, and no branchiie are yet developed. The heart is a 

 mere contractile vessel, and the blood is a clear fluid without corpuscles. At first the larvae do 

 not feed, but merely grow at the expense of the yelk, which gradually diminishes. 



"Within three or four days after hatching, the length has increased by about half the original 

 dimensions, the yelk has disappeared, the cartilaginous skeleton appears, and the heart becomes 

 divided into its chambers; but the young fish attains nearly double its first length before blood- 

 corpuscles are visible. 



"By the time the larva is two-thirds of an inch long (a length which it attains one month 

 after hatching), the primitive median fin is separated into dorsal, caudal, and anal divisions, but 

 the ventral tins have not appeared. About this period the young animal begins to feed on small 

 crustacea; and it grows so rapidly that at two months it is one and a quarter inches long, and at 

 three months has attained a length of about two inches. 



"Nearly up to this stage the elongated, scaleless little fish retains its larval proportions; but 

 in the latter part of the third mouth the body rapidly deepens, the scales begin to appear, and 

 the larva passes into the 'imago' state, that is, assumes the forms and proportions of the adult, 

 though it is not more than two inches long. After this, it goes on growing at the same rate 

 (eleven millimeters, or nearly half an inch) per month, so that at six months old it is as large as 

 a moderate-si/ed sprat . 



"The well-known ' Whitebait' of the Thames consists, so far as I have seen, almost exclusively 

 of Utrrings under six months old, and as the average size of Whitebait increases from March 

 and April onward until they become suspiciously like sprats in the late summer, it may be con- 

 cluded that they are the progeny of Herrings which spawned early in the year in the neighborhood 

 of the estuary of the Thames, up which these dainty little fish have wandered. Whether it is the 

 general habit of young Herring, even of those which are spawned in deep water, to migrate into 

 the shallow parts of the sea, or even into completely fresh waters, when such are accessible, is 

 unknown. 



"In the 'Report on Trawling' (1863) we observe: 'It is extremely difficult to obtain any sat- 

 isfactory evidence as to the length of time which the Herring requires to pass from the embryonic 

 to the adult or full condition. Of the fishermen who gave any opinion on this subject, some con- 

 sidered that a Herring takes three, and others that it requires seven, years to attain the full or 

 spawning condition; others frankly admitted that they knew nothing about the matter; and it 

 was not difficult, by a little cross-examination, to satisfy ourselves that they were all really in this 

 condition, however strongly they might hold by their triennial or septennial theories. Mr. Yarrell 

 and Mr. Mitchill suppose with more reason that Herring attain to full size and maturity in about 

 eighteen months. It does not appear, however, that there is any good evidence against the sup- 

 posit ion that the Herring reaches its spawning condition in one year. There is much reason to 

 believe that the eggs are hatched in, at most, from two to three weeks after deposition, and that 

 in six to seven weeks more (that is, at most, ten weeks from the time of laying the eggs) the young 

 have attained three inches in length. Now, it has been ascertained that a young smolt may leave 

 a river and return to it again in a couple of months increased in bulk eight or ten fold, and as a 

 36 F 



