ON THE EMBRYOLOGY OF THE TELEOSTS. 189 



The small size of the eggs and their great contraction in any hardening medium (osmic, 

 chromic, and Kleinenberg's picric acids, bichromate of potash and alcohol) prevented the cut- 

 ting of any satisfactory sections though many attempts were made. It was also found very 

 difficult to keep the eggs alive for twenty-four hours, and to these two causes must be 

 attributed the fragmentary condition of these notes, of whose shortcomings no one can be 

 more cognizant than the writer. Still in many ways, notably in witnessing the invagina- 

 tion of the hypoblast and the formation of the notochord, the extreme transparence of 

 the eggs rendered our information on these points far more satisfactory than any sections 

 could have done, as every step of the process could be clearly seen. 



In the following pages the development of the egg will be taken up by stages, each of 

 which is characterized by some prominent feature or by the appearance of some important 

 organ. From the fact that the several portions of the body are undergoing development 

 at the same time, a perfect chronological arrangement in treating of the subject cannot be 

 maintained, but it is hoped that the general features may be followed in nearly their 

 proper order. 



These stages may be briefly indicated and epitomised as follows : 



I. The maturation of the ovum. We have been unable to obtain anything on this point 

 in our eggs, but introduce some observations upon the eggs of the " Old England Hake," 

 Merlucius. 



II. The phenomena of segmentation until the formation of the germ layers. 



III. The formation of the three primary layers, the segmentation cavity, the invagina- 

 tion of the hypoblast, and the appearance of nuclei in the intermediary layer of Van 

 Bambeke. 



IV. The formation of the notochord and neural cord. During this stage the invagina- 

 tion is completed. 



V. The formation of the optic bulbs and the segmentation of the muscle plates into 

 proto-vertebrae. During this stage the first appearance of what we call " Kupffer's vesci- 

 cle " and what Balfour regards ('81 p. 61) as the post-anal vesicle, is seen. In the 

 later portion of this stage the splitting of the mesoblast into somatoplure and sjxlanchno- 

 plure begins, while the epiblast in the cephalic region thickens to form the lens of the 

 eye. 



VI. In this stage the ears and nasal pits make their appearance and undergo a portion 

 of their development; the lens of the eye is segmented from the epiblast and the first 

 traces of blood vessels were seen; the segmentation of the muscle plates still continues. 



VII. The heart and pericardial cavity begin to be differentiated and the former to 

 beat. The blastoderm at this stage completely envelopes the deutoplasm and in subsequent 

 stages will be spoken of as the yolk sac. The gills bud and the gill-arches and gill- 

 arteries appear in the later portion of this stage. In this stage the first contractions and 

 movements of the embryo are seen; simultaneously with the first beating of the heart. 



V1IL During this stage the development of the organs previously outlined progresses 

 while the outgrowth of the tail and the formation of the anus are the new features. 

 The fore and hind gut also become prominent and the lumen in the latter is readily seen. 



IX. This stage is characterized by the hatching of the embryo and is reached in from 

 forty-eight to fifty-six hours after the first segmentation furrows make their appearance, a 



