258 BERTRAM G. SMITH. 



prevented either by a rotation of the nucleus during the resting 

 stage, or by a rotation of the spindle in the early stage of mitosis. 



(3) In certain abnormal cleavages the double nuclei are really two 

 entirely separate nuclei lying side by side within a single cell. 



(4) In each of the germ-nuclei, before they come into contact, 

 there is a single nucleolus; these nucleoli disappear in the pro- 

 phase of the first cleavage, but in the succeeding telophase a 

 single nucleolus generally appears in each half of each daughter- 

 nucleus. The same is true of the succeeding cleavages, so that 

 each nucleus throughout the cleavage usually has two nucleoli 

 in the telophase or early resting stage. 



Beard (1902) described a double structure of the resting stages 

 of the nuclei of the early germ cells of Raja batis; these were not 

 traced earlier than a late gastrula stage, but influenced by the 

 findings of Hacker and Riickert, Beard did not hesitate to inter- 

 pret the double nuclei as consisting of distinct maternal and 

 paternal halves. 



Jenkinson (1904) gives some interesting figures of the fertiliza- 

 tion and first cleavage stages of Axolotl. The germ nuclei meet 

 without fusion, and the chromosomes appear separately in each 

 pronucleus while the nuclear membranes are still intact. In some 

 cases at least, these two chromosome groups remain distinct in 

 the equatorial plate after the dissolution of the nuclear mem- 

 branes. Scant attention is paid to these features in the text 

 of Jenkinson's paper, which is concerned with other matters, 

 but the author states that he has found two distinct sets of 

 chromosomes in some preparations of the fertilization spindle 

 of Triton. 



In 1904 Moenckhaus described the independence of the ma- 

 ternal and paternal chromosome groups in the early cleavage 

 spindles of the hybrids produced by fertilizing the eggs of Fundu- 

 lus with Menidia sperm. The difference in the size and shape of 

 the chromosomes of the two species makes the identification of 

 the maternal and paternal chromosomes in the case described a 

 matter of certainty. 



Pinney (1918) found that two nucleoli are typically present 

 in the nuclei of normal Ctenolabrus blastoderms, and cited evi- 



