1 88 BULLETIN OF THE BUREAU OF FISHERIES. 



LATER DEVELOPMENT. 



After the formation of the embryonic area the embryo soon becomes well outlined. 

 Plate XVIII, figure 5, illustrates a stage at which the tail bud has already grown out and 

 the anlage of the neural axis is apparent throughout the entire length of the embryo. 

 The optic vesicles are well formed and from 3 to 4 somites are already apparent. 



Plate XVIII, figure 6, represents an embryo in which the divisions of the brain are 

 becoming distinctly outlined. The auditory vesicles and from 12 to 14 somites are 

 already present. At this stage the heart is becoming differentiated as a simple curved 

 tube. The heart soon begins to pulsate, and a circulation is set up over the surface 

 of the yolk. This circulation is at first slow and irregular but soon becomes very 

 vigorous. 



The growing embryo lies in a groove in the surface of the yolk and is inclosed only 

 by the ovarian follicle. As development advances the ovarian follicle increases in size 

 and becomes increasingly vascular. The space between the egg and the follicle becomes 

 filled with a transparent fluid. Thus the embryo lives in a fluid medium. Although 

 the ovarian follicle becomes highly vascular, a placental or pseudoplacental relation- 

 ship such as exists in the selachians or even in some of the viviparous teleosts is not 

 suggested. The embryo develops no structures which would seem to be adapted to 

 absorb nourishment from a fluid medium. Furthermore, no faeces of any kind are ever 

 observed in the follicle. The abundant yolk supply in the egg is, doubtless, sufficient 

 to supply all the food material required by the embryo. 



It is probable, as was suggested by Ryder (1885), that "the very intricate meshwork 

 of fine vessels which covers the follicle supplies the developing foetus with fresh oxygen, 

 and also ser\'es to carry off the carbon dioxide in much the same way as the placenta or 

 afterbirth performs a similar duty for the young mammal developing in the uterus of 

 its parent."" The analogy between the intra-follicular respiration of the developing 

 embryo of Gambusia and the intra-uterine respiration of the young mammal must, how- 

 ever, not be carried too far. The embryo of Gambusia develops gills which apparently 

 become functional very early. An examination of the gills of an advanced embryo 

 removed from the ovarian follicle, as Ryder has already observed in the paper quoted 

 above, shows that the gill filaments are already pinnate and that the pinnae contains 

 loops of blood vessels. This condition of the gill filaments, as is well known, is not 

 attained by the larvae of many oviparous fishes for a considerable interval after hatching. 

 Furthermore, rythmical breathing movements may be observed as the embryo lies 

 coiled in the ovarian follicle. It is probable, therefore, that the intra-folHcular respira- 

 tion of the embryo of Gambusia, at least during the later stages of intra-ovarian life, is 

 more nearly analogous with the respiration of adult fishes than with the intra-uterine 

 respiration of the young mammal, the fluid in the follicle, by which the embryo is con- 

 stantly bathed, being aerated by the follicular circulation. 



As the embryo grows, the tail extends posteriorly partly encircling the egg. Soon, 

 however, it bends indifferently to the right or to the left. (PI. xviii, fig. 8.) This bending 

 brings the tip of the tail into proximity with the head. Consequently, as the caudal fin 

 is developed it overlaps the face of the embryo, sometimes partly or completely covering 

 one or both of the eyes. (PL xix, fig. 9.) 



«» Ryder, John A.: On the development of viviparous osseous fishes. Proceedings U. S. National Museum, vol. vm. p. 147. 



