Development of the Food-Fishes. 457 



in which the subnotochordal trunks are developed show an 

 abundance of nucleated cells, of large size and spherical form 

 (becoming polyhedral in microscopic preparations), filling up 

 the lumen of each vessel. Those which crowd the vena verte- 

 bralis are strongly held by one observer * (K. F. Wencke- 

 bach) to be the original form-elements of the blood. Pre- 

 cisely similar cells, rounded, colourless, and nucleated, com- 

 pletely fill up the lumen of the aortic trunk. If Wenckebach 

 be right, his conclusion must be extended, and the unde- 

 tached cells in the aorta must be also regarded as original 

 blood-cells, which have not yet acquired the colour and other 

 characteristics of blood-corpuscles. In several series of the 

 embryos of Gastrosteus spinachia at the St. Andrews Marine 

 Laboratory the passage into the heart of corpuscles, detached 

 from the yolk-cortex or periblast, was observed on many 

 occasions f, and it is highly probable that these are hsemal 

 form-elements ; but further observations are needed. No 

 perivitelline circulation, such as is seen in Gastrosteus, Gottusj 

 Liparisj etc., was observed in any of the advanced embryos 

 studied, though a branchial subnotochordal, caudal, and in 

 some cases a coeliac circulation was active. Thus minor 

 differences doubtless obtain in the development of the blood- 

 corpuscles in various Teleosteans. 



Diagnostic Features. 

 (1) Ova. — It was pointed out on a preceding page that the 

 ova, as well as the early embryos, of the species under con- 

 sideration are remarkable for the few external points of diiFer- 

 ence which they present. Their identification is often a task 

 of considerable difficulty, and even familiarity with the 

 various ova does not entirely remove the uncertainty of deter- 

 mination. Hence the desirability of establishing reliable 

 points of difference. Arranging the ova in the order of size, 

 which is a distinctive character sufficiently well marked to 

 serve for determination in the laboratory, the following fea- 

 tures may be noted : — 



Pleuronectes iilatessa : diameter '065 to "069 of an inch. — 

 The largest ovum of the various species treated of in this 

 paper. Form spherical, hyaline capsule denser than in 

 the two species of Pleuronectes mentioned below. The 

 embryo shows pigment at an early stage of a pale yellow 

 tint, quite distinguishable from P. flesus. 



* Journ. of Anat. and Physiol, vol. xix., April 1885, p. 231 : Wencke- 

 bach, " Development of Blood-corpuscles " (Perca). 



t Ann. & Mag-. Nat. Hist., Dec. 1885, p. 494 ; aud U. S. Fish. Comni. 

 Rep. 1882, p. 543. 



