306 



ME. G. BROOK ON SOME POINTS IN 



sudden rise of temperature will often kill the eggs ; and it is a 

 fact well known to pisciculturists that a greater percentage of 

 ■the ova of these forms is hatched out successfully when the tem- 

 perature is kept low. 



How far, then, a summer-breeding fish requires a high tem- 

 perature and a winter-breeding fish a low one depends doubtless 

 on features which have been adopted owing to the influence of 

 the environment. There is still, however, a certain range of 

 temperature to which each developing embryo can accommodate 

 itself according to its kind ; and I cannot help thinking that until 

 we know more of the influence of this varying range, we shall 

 not be able to compare satisfactorily the development of one fish 

 with another. 



DESOEIPTION OF THE PLATES. 



Lettering used throughout the Plates. 



a. = archiblast. 



a. s. — auditory sacs. 



b. = clear glandular body (pro- 

 bably tbe gall-bladder), referred 

 to in text, p. 303. 



b. c. = breathing-chamber. 

 br, = branchial arches. 



c. = cerebellxom. 



cc. = loose cells and nuclei lying 

 on the floor of the segmentation - 

 cavity. 



c. f. = caudal portion of the con- 

 tinuous embryonal fin. 



e. s. = embryonal shield. 

 /. b. = fore brain. 

 h. = heart. 



ht/. = hypoblast (true) or secon- 

 dary entoderm. 

 h. b. = hind brain. 

 K. V. = KupfFer's vesicle. 

 I. = liver. 

 I. j. = lower jaw. 

 m. = mouth. 

 msb. = mesoblast or mesoderm. 

 m. b. = mid brain. 



mn. = mesenteron. 



m. 0. = meduUa oblongata. 

 n. = notocord. 



M. p. = nasal pit. 



o. g. = oil-globule. 



opt. = optic lobes. 

 p. = periblast (parablast) or 

 nuclear zone. 



p. s. = pericardial sinus. 



p. e. = primary entoderm or hypo- 

 blast. 



p.f. = pectoral fin. 



p. s. = pigment-spots. 



p.v. = protovertebra, 



py. = pyloric constriction of the 

 intestine. 



s. b. = swim-bladder. 



s. c. = segmentation-cavity (both 

 before and after its extension 

 around the yolk). 



M. V. = urinary vesicle. 



U.J. = upper jaw. 



v. f. = ventral fin. 

 y. = yolk. 



s:. r. = zona radiata. 



The forming cartilages of the jaws are shown as dotted tissue in figs. 9, 9 a, 

 10, & 11. 



The details o| the alimentary tract are more hidden by pigment than is shown 

 in figs. 10 & 11. 



The direction of the circulation is indicated by arrow-heads in fig. 13. 



