482 EEPORT OF COMMISSIONER OF FISH AND FISHERIES, [28] 



other eggs of the same lot after four to six hours the spindle was still 

 present, and the germ had aggregated and acquired an unusual thick- 

 ness. Whether eggs in the two last mentioned conditions are still ca- 

 pable of impregnation I do not know, since in each instance the oppor- 

 tunity was wanting to make the test. In the cases in which the germ 

 was developed after four hours and the polar l)ody extruded, I never 

 was certainly assured that I saw a nucleus — egg-nucleus. The aggre- 

 gation of the germ, the extrusion of the polar body, and the disappear- 

 ance of the spindle are phenomena which may occur independently of 

 each other and of impregnation. Why a portion of one lot of eggs should 

 remain unchanged in water without spermatozoa, while others pass 

 through the changes above described, it is not easy to say; perhaps it 

 is, that the most mature ova pass through the stages already described, 

 while those not so mature remain unchanged, though capable of being 

 impregnated." 



6. — The ORiaiN of the yelk hypoblast. 



This structure, in its relation to the genesis of the blood, is of the 

 greatest interest physiologically, and the evidence of its true character, 

 which has been gradually accumulating in my hands, is of the most 

 conclusive nature. It has been named the parabJast bj' Klein, coiiche 

 hwmatogene by Vogt, couche intermediaire by Van Bambeke, couche cort- 

 icale, germinal layer, germinal pellicle, Dotter-haut, etc., but these terms 

 are thoroughly synonymous, and their significance need not trouble 

 us any further. I have followed its history through the later stages of 

 development to its complete disappearance, and am delighted to be able 

 to add the remarkable observations of Prof. C. K. Hoffmann* upon the 

 origin of the free nuclei in it, as observed by him in Scorpama. Ziegler 

 has also studied its development in Salmo salar, so that the evidence 

 as to the role it plays in development is almost complete. Further dis- 

 cussion of it in this essay will be postponed until I come to consider 

 the history of the blood-vascular system of the yelk-sack of several 

 forms. Under the head of the structure of the egg we have already 

 described its main features as found in several species. Hoffmann's 

 observations as to its early history are as follows, and were made upon 

 the ova of Scorpcena, Julis, etc.: 



"In respect to the segmentation I can communicate the following: 

 While the male and female pronuclei are blending to form the first 

 segmentation nucleus, the latter has already begun to again become 

 spindle-shaped. The newly-formed spindle lies in the germ with its 

 axis in conformity with the axis or diameter of the egg, and stands 

 vertically against the end of the miciopylar canal. The granules of 

 the germ tlieu group themselves in distinct rays around both poles of 

 this spindle. The well-known caryokinetic phenomena then occur, 

 which manifest themselves during each and every cleavage, and after 



* Zoologischer Anzeiger, 18S0, pp. 632-G34, 



