454 SUMMARY OF CURRENT RESEARCHES RELATING TO 



appear as though ganglion and muscle segment corresponded, yet the 

 correspondence is never complete. Further, the mesohlast of the tail is 

 represented only by the one-layered muscle band ; there is no splitting 

 of mesoblast, and therefore no space corresponding to the vertebrate 

 enterocoel. This the author regards as decisive evidence against the 

 hypothesis that the muscle blocks in the tail of the Appendicularidae 

 represent the primitive segments of Amphioxus. Indeed, he considers 

 that the muscle bands are essentially unsegmented structures, the appear- 

 ance of segmentation being the result of the action of reagents which 

 render the cell-boundaries conspicuous. Further, he is of opinion that 

 the evidence is decidedly against the view that the structure of the tail 

 can be explained by the hypothesis of descent from distinctly segmented 

 forms. If therefore the Appendicularidaj are to be regarded as the most 

 primitive of the Tunicates, then the relation of Tunicates to Vertebrates 

 must be much less intimate than is generally supposed ; for while the 

 ancestral Tunicate had neither distinct segmentation nor an enterocoelic 

 body -cavity, the ancestral Vertebrate must have had both, and divergence 

 must therefore have occurred at a very distant period. 



INVERTEBRATA. 



Mollusca. 

 y. Gastropoda. 



Maturation and Fertilisation in Pulmonata.* — Mr. H. E. Linville 

 has studied these processes in two species of Limax and two species of 

 Limnsea. He finds that the centrosome and the centrosphere are ex- 

 tremely variable structures, both in size and in their reaction to stains. 

 In the processes leading to the formation of the first polar cell in Limax 

 maximus, no centrosome is visible ; the astral rays apparently begin in 

 the thickened wall of a pale centrosphere. After the formation of the 

 first polar cell, and also after the formation of the second, the centro- 

 sphere is greatly enlarged in Limax maximus and Limnsea elodes. In 

 the latter there is great variation in the condition of the centrosome and 

 centrosphere. Two facts suggest that the centrosphere is not a per- 

 manent organ of the cell : — First, the centrosphere is sometimes invisible, 

 on account of the increased area at the centre of the aster which reacts 

 to the stain, and, secondly, astral rays beginning in the centrosome con- 

 tinue through the centrosphere, which thus appears to be no more than 

 a region of thickening in the astral fibres. The centrosome in the first 

 maturation spindle of Limnsea varies from a very minute granule to a 

 diameter at least as great as the transverse dimension of the spindle 

 itself ; in the second maturation spindle it was never large. In both 

 Limax and Limnsea, after the formation of the second polar cell, the dis- 

 appearance of the egg-centrosome and the egg-aster, though long delayed, 

 is complete. 



A reduction division of the chromosomes in the Eoux-Weismann 

 sense is seen in the second maturation division of Limnsea elodes, when 

 there is a transverse division of the dyads resulting from the longitudinal 

 splitting of the masked tetrads of the first maturation division. 



In fertilisation the tail follows the sperm-head into the egg, but is 



• Bull. Mus. Harvard., xxxv. (1900) pp. 213-48 (4 pis.). 



