208 KINGSLEY AND CONN 



side of the same central organ, the condition which obtains in the adult fish. During the 

 differentiations thus described other changes take place, and before the stage represented 

 in fig. 42 is reached the lumen in the brain, optic bulbs, and neural cord appears as shown 

 in figs. 36 and 45. 



Synchronous with the differentiation of the regions of the brain, the mesoblast adjoining 

 the notochord (fig. 32) becomes separated from the rest of the layer as the muscle plates, 

 and with the separation of the optic bulbs the muscle plates are divided into protoverte- 

 brae. The first of these protovertebrae to be formed is at about the middle of the body, 

 and the fissures which limit it arise simultaneously. They begin close to the notochord 

 above and progress outward and downward and at the same time slightly backward. 

 From this central one the formation of the protovertebrae extends gradually in both direc- 

 tions. At the same time the formation of pigment cells is seen. These arise as black 

 dots and gradually increase and change their shape until at last they assume forms like 

 those shown in fig. 51. 



Regarding the peculiar structure which from its first describer we may call Kupffer's ves- 

 icle I have but little to say. It first makes its appearance just before the stage shown 

 in fig. 34 on the under surface of the posterior end of the embryo and rapidly increases in 

 size until it acquires a diameter nearly equal to that of the fish. Of its subsequent fate 

 our notes afford no information. Its first appearance is indicated by one or two small 

 globules which soon are joined by others until an appearance like that of fig. 52 is seen. 

 Very soon these globules unite and in two hours the vesicle has the appearance and 

 relative proportion of fig. 53. Fig. 54 shows about the limit of its development and is 

 reached in about five hours from the first appearance of the minute globules. 



As mentioned above, this vesicle was first noticed by Kupffer ('68) and by him regarded 

 as a rudimentary allantois. Balfour ('81) regards it as homologous with the terminal vesi- 

 cle of the post-anal gut of Elasmobranchs. On the other hand Henneguy ('80) studying 

 the perch thinks he saw an opening or traces of invagination of the vesicle and would 

 homologize it with the primitive intestine of the Cyclostomi and Batrachia and its opening 

 with the anus of Rusconi, a view which it seems to me is entirely unwarranted by the pre- 

 vious growth of the embryo and by the method of origin and position of the vesicle as we 

 have seen it. The view of Balfour seems much more probable. 



The epiblast over the optic lobe begins to thicken to form the lens of the eye when six- 

 teen protovertebrae are outlined. This thickening increases until soon it acquires the 

 character shown in fig. 49 and almost immediately the thickening begins to be segmented 

 off and to make its way into a depression in the optic lobes and to acquire more and more 

 of a spherical character as shown in figs. 55-57. The manner of the involution of the lens 

 and the features connected with it have so often been described that it is unnecessary to 

 repeat them here. 



At about this time the mesoblast begins to split into somatopleure and splanchnopleure. 

 This splitting begins at the head end of the embryo and progresses regularly in every 

 direction. This is a true splitting and not as suggested by Mr. Ryder in a letter to the 

 author an apparent one. He interprets his observations as follows : the portion of the 

 mesoblast which extends down over the yolk is that portion which eventually forms the 

 splanchnopleure and that the somatopleure gradually extends down between this layer and 



