294 bulletin: museum of comparative zoology. 



establish conclusively the occurrence of a numerical reduction of the 

 chromosomes during the maturation of the egg. He showed that the 

 egg nucleus contains only half the number of chromosomes normal to 

 the species, and that after the fusion of the germ nuclei the com- 

 plete somatic number once more appears in the formation of the first 

 cleavage nucleus. He was, furthermore, the first to count the chromo- 

 somes in any coelenterate. 



Further details of the process of maturation were soon after presented 

 by Brauer ('91 a ), whose conclusions in the main substantiate for Hydra, 

 the earlier work of Boveri ('90). According to Brauer, the division of 

 the chromosomes in the first maturation mitosis is apparently transverse. 

 To Brauer ('91 b ), furthermore, we owe the discovery that in Tubularia, 

 at least, the sperm nucleus is accompanied during its journey through 

 the egg by a distinct astral radiation. 



Boveri's ('90) discovery of the numerical reduction of the chromo- 

 somes in Tiara was supported by Hacker ('92 a ), who was able to count 

 six such structures in the second maturation spindle, and twelve in the 

 first cleavage spindle of Aequorea. In this form the nucleolus is cast 

 out of the nucleus before maturation, and long persists in the yolk, — 

 a fact observed also by H. V. Wilson ('94) in the sponges Hircinia and 

 Taedanione. Hacker ('92 a ) further states that the sperm nucleus is 

 accompanied by an aster, and notes the fact, previously detected by 

 Brauer ('91 b ), that while the cleavage spindles have large and promi- 

 nent asters, the mitotic figures in older larvae show no trace of any such 

 structures. 



We owe to Maas ('99) the only detailed description of the matura- 

 tion, fertilization, and cleavage of a sponge which has yet been written. 

 In Sycandi-a the chromosomes of the first polar spindle are formed by a 

 condensation of the chromatin microsomes present during the growth 

 period of the egg. The sperm nucleus consists of a chi'omatic mass, 

 together with a more refractive " middle-piece " which forms the centre 

 of a series of astral radiations. When the two germ nuclei come in 

 contact each contains sixteen chromosomes, which, after fusion, pass 

 without alteration into the first cleavage spindle. 



The same stages have been described within recent years in various 

 hydroids, by Morgenstein (: 01), Harm (:02), and Wulfert (: 02). 

 Among these, Morgenstein's (: Ol) observations on Cordylophora are 

 of most interest because they contain the first detailed study of the 

 coelenterate centrosome. No centrosome could be detected in either 

 of the maturation spindles, but the sperm nucleus, in its migration 



