208 THOMAS H. BRYCE. 



double Vs originating in the wings of tlie "equatorial 

 crown." These are separated and carried to the poles. A 

 partial reconstruction takes place in the anaphase, but Vs 

 again appear in the telophase, and the double Vs remaining 

 in the ovum are separated from one another in the second 

 polar spindle. The whole description shows a very compli- 

 cated process, and exactly what I have found in Echinus in 

 a much simpler form, because as there are no Vs or twisted 

 threads to complicate the picture, I may say that only the 

 initial stages described by Carnoy and Le Brun are found in 

 Echinus. 



This is the only positive evidence of the occurrence of a 

 simultaneous double split of the compound chromosome of 

 the heterotypical division in animals. It will be seen that 

 my interpretation agrees in the main with that of Carnoy and 

 Le Brun, and Janssens ; and further, that the same idea has 

 enabled Strasburger to reduce the heterotype in the higher 

 plants to one common plan. 



Now Echinus falls exactly into line in every essential 

 respect with another considerable series of cases recently 

 described. 



1. Prostheecraeus, Klinckowstrom, 1897. 



2. Various Polyclads, Francotte, 1897. 



3. Thysanozoon, Van der Stricht, 1898. 



4. Thalassema, Griffin, 1899. 



5. Zirphfea, Griffin, 1899. 



G. Tubifex and Clepsine, Gathy, 1900. 



In every one of these the figures belong to the same type, 

 except that Van der Stricht and Griffin describe rings in the 

 prophases. The last observer has not given details, because 

 the chromosomes were too minute for analysis. 



The first four authors all explain their results according to 

 the diagram given below (Text-fig. 1). The double rods 

 resulting from the compression of the ring are placed with 

 the longitudinal cleft in the plane of the equator of the 

 spindle, and are drawn apart by their middle points to 

 form U-shaped or V-shaped figures, and the breaking 



