HABITS, MORPHOLOGY, AND EMBRYOLOGY OF GAMBUSIA AFFINIS. 1 87 



EMBRYOLOGY. 



OVUM. 



The mature ovum is a spherical body having a diameter of about 1.8 millimeters. 

 It has a gold-yellow color and, being heavily laden with yolk, is quite opaque. It is 

 invested by no distinct egg membrane such as invests the eggs of most of the oviparous 

 fishes, but is covered only by a thin, vitelline membrane. Beneath the vitelline mem- 

 brane the entire surface is more or less completely covered by oil globules of unequal 

 size and distribution. (PI. xvn, fig. i.) 



BLASTODERM. 



The ova are fertilized within the ovarian follicles. Unless the time of fertiUzation 

 can be controlled, it becomes difficult to secure the earliest stages of development. The 

 earliest stages which were secured after fertilization showed a small blastoderm in the 

 many-cell stage. This blastoderm appears as a small, almost circular cap of cells which 

 is slightly elevated above the surface of the yolk. (PI. xvii, fig. 2, B.) The distribu- 

 tion of the oil globules is not disturbed during the process of cleavage and numerous 

 globules may be observed through the blastoderm. 



As the blastoderm increases in size, the cleavage cavity becomes plainly visible. 

 The germ ring is never well defined, but appears as a slight thickening of the periphery 

 of the blastoderm. The cleavage cavity, as observed through the overlying blastoderm, 

 soon assumes a somewhat triangular outline. The blastoderm becomes slightly 

 elongated along the axis, which becomes the future axis of the embryo. At the side of 

 the cleavage cavity on which the thickened area at the periphery of the blastoderm is 

 broadest, the blastoderm becomes thicker and more opaque. This area is symmetrically 

 divided by the long axis of the blastoderm and, inasmuch as it gives rise to the embryonic 

 shield, may be recognized as the posterior pole of the blastoderm. This area increases 

 in size and distinctness until the embryonic shield is well outlined. (PI. xvn, fig. 4.) 



DIFFERENTIATION OF THE EMBRYO. 



From the posterior pole of the embryonic shield a narrow thickened area grows 

 anteriorly. This thickened area alone represents the embryonic area, while the thinner 

 lateral areas represent the extra-embryonic area of the embryonic shield. (PI. xvii, fig. 4.) 

 The embryonic area continues to grow anteriorly over the cleavage cavity and becomes 

 gradually enlarged at the anterior end. In this manner the head of the future embryo 

 becomes outlined. 



While the embryonic area is becoming differentiated the blastoderm spreads rapidly 

 pver the yolk until the latter is completely covered. The progress of the growth of the 

 blastoderm over the yolk could not be observed satisfactorily, partly because the germ 

 ring is not well defined and not easily observed on this very opaque egg and partly 

 because not all the desired stages of development could be secured. A careful study of 

 the stages available, however, seems to indicate that the differentiation of the embryo 

 of Gambusia takes place in a manner which is quite typical for teleosts. 



