[29] EMBRYOGRAPHY OF OSSEOUS FISHES. 483 



some minutes the spindle-shaped stage has again vanished and two 

 new nuclei have been formed, both of which lie in the plane of the axis 

 or diameter of the egg. One of them lies at about half the depth of the 

 axis of the germ, the other deeper and nearer the yelk. With the di- 

 vision of the first segmentation nucleus into two new nuclei, the con- 

 sequent cleavage has led to the division of the egg into two very unequal 

 portions, the upper and smaller lying near the micropyle and consisting 

 of protoplasm, which, at the level of half the height of the axis of its 

 substance, the germ, contains a nucleus; this portion I shall call the 

 archiblast [the germinal disk of this essay], the other ver^^ much larger 

 portion, the parablast [the yelk hypoblast; germinal layer]. The par- 

 ablast consists indeed for the greater part of food-yelk, but it ap- 

 proaches in character the germ, the protoplasm in which lies a nucleus 

 close to the yelk, and is continued over the whole yelk as a thin envelope. 

 The archiblast [germinal disk] only segments; its nucleus is the parent 

 of all the cleavage nuclei; the parablast [germinal layer] does not seg- 

 ment; nuclear division only takes place in it; it is developed into a 

 multinucleolar cell. 



"Before the separation of the archiblast from the parablast, each nu- 

 cleus of both these parts has been transformed into a new caryokinetic 

 figure or spindle, the position of the axes of which are at right angles 

 to the axis or diameter of the egg. The spindle or caryokinetic figure 

 formed by the nucleus of the archiblast (germinal disk) is a magnificent 

 spectacle; that of the parablast is less distinct on account of its more 

 central position. Before two new nuclei have been formed from the 

 spindle in the archiblast the first segmentation furrow begins to divide 

 the archiblast into two equal sized parts. Both are divided from each 

 other by this furrow, but at their bases or lower surface they are still 

 continuous with the underlying parablast. Each nucleus of the two 

 portions of the archiblast soon begins to prepare for another division, 

 and in the parablast two free nuclei may be observed, which are also 

 getting ready to divide. At the same time the separation of the archi- 

 blast frooi the parablast, at the base of the former, begins to manifest 

 itself, and when the archiblast has been divided into four segments 

 they have become quite free, in that they have now separated them- 

 selves from the parablast below. They then lie on the protoplasmic 

 layer of the latter, and in this layer four free nuclei may now be noted. 

 The cleavage or segmentation now proceeds regularly. When the nu- 

 clei of the archiblast have been transformed into new spindles, the same 

 takes place with the nuclei of the parablast. All of the free nuclei are 

 ever undergoing similar phases of division, or are synchronously in 

 a state of rest, and the nuclei of the segments of the archiblast pass 

 through the same phases, at least duriug the first few hours of segmen- 

 tation, whilst at a later period the free nuclei of the parablast pass into 

 a resting stage. 



"With the comi^letion of segmentation, the Teleosteau egg consists of 



