[19] EMBRYOGRAPHY OF OSSEOUS FISHES. 473 



a coating over the yelk wliile the egg is developiug within its membrane 

 or capsule, and even includes the yelk after the blastoderm has closed 

 over it. This yelk membrane or pellicle derived from the germinal mat- 

 ter forms the floor of the segmentation cavity. In the substance of the 

 membrane free nuclei also develop; these doubtless contribute by a pro- 

 cess of intussusception to the formation of hypoblastic structures at 

 an early stage of development, but later these nuclei appear to give 

 origin only to blood cells, mainly in the form of leucocytes, except in the 

 case of Tylosurus, where the blood cells of the early stages are colorless, 

 but haemoglobin is soon developed, and finally, it would appear, con- 

 currently with their free germination from the membrane they are al 

 ready colored. The free nuclei are most numerously imbedded in the por- 

 tion of the yelk membrane or hypoblast in the neighborhood of the heart, 

 near the head, and at the anterior portion of the yelk-sack. 



5. — ^Impregnation of the egg. 



Of the history of this process in the Teleostean egg we as yet possess 

 very few trustworthy observations, except those of C. K. Hotfraann. 

 This is in part due to the difficulty attending its study in an egg of such 

 a disproportionately large size as that of the fish with its large yelk, 

 and no observations have yet been made upon this point with svich suc- 

 cess as upon the eggs of moUusks and echinoderms, as, e. g., Limax cam- 

 pestris, by E. L. Mark; Asterias glaciaUs, by Fol ; and Toxopnenstes varie- 

 gatus, by Selenka. Hertwig, Flemming, and Bambeke have also contrib- 

 uted essentially to our knowledge of the process of impregnation, the 

 former and latter especially in relation to what occurs in the amphibian 

 ovum. Almost all observers seem to be agreed that a single spermato- 

 zoon only is requisite. This enters the egg either through a penetrable 

 membrane or through a micropyle, and blends at once with the plasma 

 ,of the egg, i)roducing in this process of coalescence a clear space, sur- 

 rounded by granular rays in the vicinity, which has been designated the 

 male pronucleus. This male pronucleus then blends with the female 

 pronucleus to form the first segmentation nucleus, but the origin of the 

 female pronucleus, as described by Fol, is somewhat complicated. It 

 is derived from the germinative vesicle by a complex metamorphosis of 

 the latter. The following is the series of events attending impregna- 

 tion and the order of their occurrence, essentially, as observed by Fol 

 and summarized by Balfour : 



"1. Transportation of the germinal vesicle to the surface of the egg. 



" 2. Absorption of the membrane of the germinal vesicle and metamor- 

 phosis of the germinal spot and nuclear reticulum. 



"3. Assumption of a spindle character by the remains of the germi- 

 nal vesicle, these remains being probably in part formed from the germi- 

 nal spot. 



"4. Entrance of one end of the spindle into a protoplasmic prominence 

 at the surface of the egg. 



