hargitt: pennaria tiarella and tubularia crocea. 179 



Polyspermy is of frequent occurrence. The sperm head begins its 

 transformation into the sperm nucleus soon after entrance, and while 

 close to the surface of the egg. The egg nucleus is often composed of 

 several distinct vesicles, and the sperm nucleus occasionally appears 

 lobed, as though formed by the confluence of several vesicles. An 

 aster may accompany the sperm nucleus, or it may perhaps be in 

 connection with the egg nucleus, but at the time of apposition of the 

 nuclei no astral radiations are present, as a rule. A complete fusion 

 of the nuclei may occur, or they may retain their independence while 

 the first cleavage spindle is forming. Chromosomes seem to take 

 their definitive form only after the first cleavage spindle is present. 

 The origin of the cleavage asters could not be determined. 



4. Cleavage. — Figure 27, which shows the first-cleavage spindle 

 forming, gives the impression of its formation from the cytoplasm, 

 since the nuclear membrane, still intact, is deeply indented opposite 

 the aster, the astral rays extending into the indentations. The second 

 cleavage spindle (Fig. 28) gives evidence of the same thing, for although 

 the spindle fibres are nearly formed, and the elongated nucleus is in 

 the axis of the spindle, the nuclear membrane is still unbroken. Large 

 conspicuous asters occur at the poles of the second cleavage spindle; 

 the radiations do not enter the large clear centrosphere, nor is there a 

 central body found in it. The chromatin of the nucleus is beginning 

 to produce beaded strands, though definitive chromosomes are not yet 

 formed, and a part of the reticulum remains. In this egg, although 

 the second spindle is forming, the first cleavage furrow is only started, 

 and in other eggs when the third division of the nucleus had begun 

 the first cytoplasmic division was still unfinished, and the second 

 cleavage furrow only just started. This delay in the cytoplasmic 

 division was found by Hargitt (:04°) to be common in Pennaria, and 

 we shall see that it is also typical in Tubularia. Further cleavage 

 stages of Pennaria have not been studied. 



B. Tubularia crocea. 



1. Oogonia.— The primordial germ cells divide mitotically to 

 form the oogonia, which are so closely packed around the spadix that 

 their outlines are obscured, if not wholly obliterated, as has been de- 

 scribed by Allen (:00). The nuclei of the oogonia are relatively large 

 and each contains a single large nucleolus, as Allen has also observed. 

 The nucleolus stains intensely in iron hematoxylin, but in hematoxylin 

 and eosin it selects the acid or plasma stain. Delicate linin fibres 



