190 THOMAS H. BRYrE. 



archoplasmic area divides, and the two parts being drawn 

 asunder, a spindle is spun out between them, which moves 

 tangeutially over the nucleus. As it grows the spindle-fibres 

 project into the vesicle, the nuclear membrane is dissolved, 

 and the spindle then rotates to become the first polar 

 amphiaster. In Haecker's text-book (p. 123) tbe process in 

 the living egg is described in much the same fashion. A 

 clear area is developed between the remains of the germinal 

 vesicle and the surface, sum^ounded by a radiation which soon 

 forms a double star, which is the beginning of the amphiaster. 



The invagination of the germinal vesicle in the egg of 

 Echinus is probably secondary. It may be related to the fact 

 that the wall of the vesicle comes exceedingly close to the sur- 

 face of tlie egg. The mounting of the vesicle to the surface 

 is a fact which, so far as I know, has not been satisfactorily 

 explained. Haecker (1893) suggested that it is due to the 

 action of gravity causing a movement in the elements of the 

 egg after the force connected with the exchange of material 

 between nucleus and cytoplasm, which keeps the vesicle in 

 the centre of the egg, ceases with full growth. My prepara- 

 tions do not throw any light on the point. 



Fig. 8 represents a somewhat oblique section of the 

 germinal vesicle at a later stage. It shows the two asters 

 arranged tangentially to the surface of the egg, but betAveen 

 them, and extending towards the surface, is a finely reticular 

 mass, out of which the delicate wavy and interdigitating rays 

 of the asters are evidently spun. Embedded in this reticulum 

 are seen the chromatin segments. At a later stage (fig. 9) 

 all these are drawn into the area between the asters, which is 

 seen now as a finely alveolar or reticular plate. Round this 

 central plate is a complicated reticulum of fibres crossing and 

 intercrossing, but on the whole radiating from the central 

 plate. In this reticular zone is also seen, at a little distance 

 from the plate, one centrosome surrounded by rays, obviously 

 part of the general reticulum. In the adjoining section a 

 second aster was present on the side of the plate removed 

 from the surface of the ovuin. At this stage one hardly ever 



