340 BULLETIN OF THE 



before the disappearance of the whole outline, and I am inclined to think 

 this case is closely related with the condition found in corresponding 

 stages in Limax, where the evidence strongly favors the view that there 

 is not an actual union of the pronuclei preceding the appearance of the 

 asters. It is noticeable that Hertwig makes no mention of the spindle 

 in this case, doubtless from its tardy appearance, as in Limax. He does 

 not seem to have discovered the asters at any point other than those 

 which lie in the plane of nuclear contact, nor that they may come into 

 view successively. In both particulars the eggs of Limax have proved 

 more favorable in determining, if not the prevailing, yet at least the 

 possible, order of the events of nuclear metamorphosis. 



In each of the conjugated nuclei arises in Pterotrachea a single, or in 

 Phyllirhoe numerous nucleoli, which are, however, of only temporary 

 duration. After their disappearance two radial figures arise, as in 

 Tiedemannia, but in the present case the partition between the two 

 nuclei disappears, so that an actual fusion takes place. In the nuclear 

 space thus made common, fine fibres stretch between the two suns. 

 Later, the contents of this nuclear vacuole, which in Hertwig's opinion 

 contains in addition to the spindle nothing but nuclear sap, become min- 

 gled with the surrounding protoplasm, whereby the comparatively small 

 spindle comes to lie free in the yolk (pp. 208, 209). 



In Pterotrachea, as well as in Mytilus, the activity of the vegetative 

 pole of the egg just before or during the first segmentation results in 

 more or less conspicuous protoplasmic elevations of the yolk. 



Very recently Bobretzky ('78") has published observations on early 

 stages in the development of insects. He maintains that the blasto- 

 derm cells in certain Lepidoptera are formed within the yolk by a pro- 

 cess of cell division, and migrate as amoeboid cells — hitherto mistaken 

 for nuclei — to the surface, where they successively make their appear- 

 ance to form the blastoderm. 



Lang's ('78) studies on the development of Balanus contain indica- 

 tions that in the segmentation the nucleus undergoes a spindle meta- 

 morphosis, but give no data concerning the maturation or fecundation, 

 save that a segregation of the ectodermal protoplasm at one pole pre- 

 cedes the " unequal segmentation." 



Hatschek ('78) has given little attention in his excellent paper on the 

 development of Annelids to the details of cell division. He says, how- 

 ever, (p. 17,) that one very often finds in the primitive cells of the meso- 

 derm indications of division, — spindle-shaped nuclei, and granular rays 

 in the protoplasm. 



