August 12, 1898.] 



SCIENCE. 



177 



follicular epithelium, which remains be- 

 hind in the ovary when the ovum passes 

 into the pouch, forms a very conspicuous 

 corpus luteum, which persists for a consid- 

 erable period. 



In the colonies studied, both the young- 

 est buds and the the adult zooids contained 

 about the same number of oogonia, so that 

 in these the whole of the ovogenesis con- 

 sists in growth and maturation only. The 

 yolk bodies, which are very large, begin to 

 be formed at the periphery of the ovum 

 when it has reached about half its final 

 diameter. At this time the germinative 

 vesicle has reached its maximum size ; it 

 has a full outline, a conspicuous stained 

 network and a large nucleolus. From now 

 on, while most of the yolk is formed and 

 the ovum acquires the last seven-eighths 

 of its ultimate volume, the germinative 

 vesicle decreases in size, until it has but 

 half its maximum diameter, acquires a 

 stellate outline and a marked avidity for 

 most stains. The shrinking of the germi- 

 native vesicle, then, is not associated with 

 maturation in this case, but with yolk 

 formation. 



The nucleolus, though usually obscured 

 by most stains, persists with little change 

 throughout this shrinking. It does not 

 form the stellate body found in the old 

 ova, as Davidoff maintained, but is found 

 within this body, which is itself the remains 

 of the germinative vesicle. The nucleolus 

 at this stage is quite complex, consisting of 

 a homogeneous cortex, an eccentric finely 

 granular medulla, and within the latter 

 several very highly refractive bodies, the 

 largest of which may have a granular ap- 

 pearance. During the greater part of the 

 growing period these refractive bodies are 

 the only substance in the germinative 

 vesicle that takes the chromatin stain with 

 a methyl green and acid fuchsin combina- 

 tion. However, shortly before the egg 

 leaves the ovary chromatin is detected in 



other regions of the vesicle, so that it is 

 probable that the tetrads are not formed 

 from any part of the nucleolus. These re- 

 fractive bodies persist even after the ovum 

 has passed into the oviduct and the rest of 

 the nucleolus, together with the germina- 

 tive membrane, can no longer be seen. 



The tetrads, of which there appear to be 

 normally twelve, are formed during the 

 passage of the egg through the oviduct. Two 

 polar cells are formed, but in neither of the 

 divisions accompanying their production 

 nor in any of the earlier cleavages has any 

 centrosome, aster or spindle fibre been 

 found, although several good preparations 

 of these stages have been obtained. In all 

 of these processes the amount of granular 

 undifferentiated cytoplasm is very small ; 

 by far the greater part of the ovum is filled 

 with yolk bodies, between which no inter- 

 stitial cytoplasm can be detected. It may 

 be that the absence of these characteristic 

 accompaniments of mitosis is due to the 

 small amount of active cytoplasm present. 



Observations on Non- Sexual Reproduction in 

 Dero vaga. (Abstract.) By T. W. Gal- 

 loway. 

 Budding may take place in any of the 



setigerous segments from the 16th to the 



21st. 



In the anterior zooid subsequent bud- 

 zones are produced in the same seg- 

 ment in which the first occurred. When 

 the posterior individual reproduces, the 

 bud-zone may be in the segment bearing 

 the same number as that of the anterior 

 zooid, or it may be in front or posterior to 

 that segment. It is thus the posterior zooid 

 which introduces the variability. In Dero 

 the relation between the normal increase of 

 segments and budding is such as to suggest 



