194 bulletin: museum of comparative zoology. 



divisions, at least up to the formation of the planula, take place by 

 mitosis. The egg presents a rather marked polarity. Segmentation 

 of the egg may follow nuclear division almost at once, though always 

 slightly delayed, or nuclear proliferation may continue for some time 

 before cytoplasmic division begins. A segmentation cavity usually 

 occurs, though it may be represented by separate spaces between 

 adjacent blastomeres. A nearly tyjjical blastula — a single layer of 

 cells surrounding a large segmentation cavity — is sometimes produced, 

 but this may be more or less modified, even to the complete obliteration 

 of the cleavage cavity. In some cases no stage exactly comparable to 

 a blastula can be discovered. The solid mass of cells, the so-called 

 "morula," which sooner or later results, is to be regarded as the out- 

 come of the process of germ-layer formation and not as the end of 

 segmentation. The germ layers are the result of a primary multipolar 

 delamination, though the definitive ectoderm and entoderm are later 

 more completely differentiated by a further division and rearrangement 

 of the cells of the early germ layers. 



Double blastulae have been found, which are probably the result of 

 an incomplete first cleavage. The nuclei of early cleavage stages are 

 usually single, those of blastulae usually consist of two vesicles. Chro- 

 mosomes arise independently but synchronously in the two parts of the 

 double nuclei, and their distinctness may or may not continue through- 

 out the mitosis. Daughter nuclei take the form of two apposed vesicles 

 which, however, remain distinct through the entire resting period. 

 When the germ layers have been definitely formed and are separated 

 by the supporting membrane, double nuclei no longer appear. 



IV. Discussion and Historical Review. 



1. Growth of eggs. — It is well known that in the Coelenterata, 

 although the primordial germ cells are all alike, only a few of the many 

 oocytes become mature, the rest serving as food for the survivors. 

 Most authors have believed that the circumstances of favorable posi- 

 tion and nourishment determine which oocytes become egg- and 

 which food-cells. Brauer ('91^) and Wulfert (:02), however, believe 

 that a differentiation occurs during the migration of the germ cells into 

 the gonophore, though the latter maintains that this early differentia- 

 tion is the result of favorable position and nourishment. Gronberg 

 ('98) and Miiller (:08), on the other hand, think the differentiation 

 occurs in the oocytes. 



