EGGS AND LARViE OF MENIDIA MENIDIA AND M. BERYLUNA. 115 



It is obvious, then, that the purpose of these gelatinous strands is to afford ready 

 means by which the eggs may attach themselves to vegetation or other objects in the 

 water. It is likewise apparent that because of this provision the parents spawn in places 

 where there is an abundance of vegetation. 



The eggs are yellowish green, as seen in a mass with the unaided eye, but when 

 seen singly under magnification they are semitransparent, and the slight greenish pig- 

 ment appears to be in the individual granules of the yolk. It is also seen that what 

 appeared to the unaided eye to be a single thread by means of which the egg becomes 

 attached is really a bundle of very fine strands of uniform size. A large fat globule, 

 occupying a central position, is always present, and smaller globules, from a few to several 

 in number, are variously distributed. There is a perceptible space between the egg 

 membrane and the vitelline membrane which varies in width. It is broadest at the germ 

 disk and narrowest opposite this point; that is, the yolk sphere occupies the upper part 

 of the egg sphere (fig. 85). 



EMBRYOLOGY. 



The protoplasm becomes concentrated after fertilization has taken place at the lower 

 pole of the egg, forming a cap on the yolk of the egg. This mass of protoplasm is the 

 blastodisk. The first cleavage plane cuts the blastodisk parallel with the axis, passing 

 through the upper and lower pole of the egg (fig. 86). The second is at right angles to 

 the first (fig. 87). Cleavage in these eggs is quite regular (fig. 88) and proceeds rapidly 

 in a relatively high temperature, but it is much retarded in a relatively low water tem- 

 perature; for example, the stage represented in figure 89 was reached in about six hours 

 in a water temperature of 84° F., but in a water temperature of about 40 F. the time 

 required to reach the same stage was approximately 48 hours. 



The outline of the blastoderm on the yolk after an advanced cleavage stage is reached 

 is only indistinctly visible, and the development can not be clearly observed. Within 

 12 hours after fertilization with a water temperature of 84 F., or within about 60 hours 

 with the temperature of the water at approximately 6o° F., the outline of the embryo, 

 however, may be seen (fig. 90). 3 It is curved with the periphery of the egg and is some- 

 what less opaque than the remainder of the blastoderm. 



Within 24 hours after fertilization with a water temperature of about 84 F., or 

 within approximately four days with a water temperature of about 6o° F., the embryo 

 is distinctly formed and has about 12 somites. It now extends at least half the distance 

 around the periphery of the egg, and only one large fat globule remains (fig. 91). 



Two days after fertilization with a water temperature of 84 F., or about seven 

 days with a water temperature of 6o° F., the heart begins to pulsate and large blood 

 vessels may be seen traversing the yolk. The blood at first contains relatively few 

 corpuscles, which flow slowly, but their number and speed increase rapidly with the 

 development of the embryo (fig. 92). Soon after circulation is well established large 

 dark-green or brownish chromatophores appear on the yolk and smaller ones on the 

 embryo. The embryo by this time has fully encircled the egg and is segmented through- 

 out and capable of considerable movement, the tail being partly free. The period of 

 incubation is about 16 days in a water temperature varying from 40 to 6o° F. 



2 The author's attention was called to an irregularity iu this figure by Dr. Albert Kuntz, who kindly examined the illustra- 

 tions and read the manuscript, suggesting that an abnormal eg*; was probably studied, as the outline of the advancing blastoderm 

 should he regular and not broken as shown in the figure. 



