n MEIOSIS IN LEPIDOSIREN 



37 



in the above account of Tomopteris, various additional features and 

 minor modifications are commonly met with in other cases, as will be 

 illustrated by short descriptions of the meiotic phases in the lung-fish, 

 Lepidosiren paradoxa, and in certain insects. 



(2) Meiosis in Lepidosiren paradoxa (Fig. 16) 



Lepidosiren is probably unsurpassed as an object for cytological 

 research, owing to the great size of its nuclei and the clear sharp outlines 

 of its chromatic elements as prepared for examination by the ordinary 

 cytological methods. Another great advantage which it possesses is 

 the fact that the chromosomes differ greatly from each other in size. 

 The number of chromosomes in the body tissues is thirty-eight, and 

 in the gamete nineteen. 



The most important stages of the meiosis of this species are depicted 

 in Figs. 16 and i6a, in which a few pre-meiotic figures are also shown. 

 Fig. 16, A, is a spermatogonia! nucleus, taken from that part of the testis in 

 which active spermatogonial mitosis is proceeding. Its coarse structure as 

 compared with that of the spermatocyte I. nucleus is to be noted. This 

 is a common and conspicuous distinction between nuclei of these two 

 grades. 



The spermatogonial prophases are of an extremely simple nature, 

 contrasting, therefore, strongly with the complicated series of events 

 which takes place in the meiotic prophase. The coarse blocks of 

 chromatin of the resting spermatogonial nucleus form themselves into 

 long threads by lengthening and fusion, and then these threads shorten 

 and thicken into the definitive metaphase chromosomes. 



The further contraction of the chromosomes in anaphase (Fig. 16, D) 

 brings to light a feature which is visible, though less conspicuous, among 

 the longer prophase or metaphase chromosomes, namely, that they are 

 of very different lengths (see also Fig. 65). Especially noticeable is the 

 pair of very long chromosomes which are bent (at this stage) into the 

 form of a V. As these chromosomes are about twice the length of the 

 next largest pair, they are easily identified whenever the chromosomes 

 are individually distinguishable. 



As in Tomopteris, syndesis begins at the polar ends of the chromosomes 

 and spreads along them from this point. It will be noticed that polar 

 views of the nuclei are shown in this figure, while the corresponding 

 figures of Tomopteris represent the nuclei seen from the side. 



The onset of the diplotene stage (Fig. 16a, H) is considerably obscured 

 in Lepidosiren by the simultaneous contraction of the greater part of the 

 chromatin elements into a compact mass, leaving the larger part of the 

 nuclear cavity free from chromatin. This is a frequent and extremely 

 characteristic feature of the meiotic phase, though it is not universal. 



