THE EARLY DEVELOPMENT OF THE M ARSL'PIATTA. 13^ 



may accordingly designate this central reticular area as the 

 deutoplasmic zone. 



If we pass noAV from the full-grown to the ripe ovarian 

 ovum (PL 1, figs. 2 and 3), i. e. an ovum in which either the- 

 first polar spindle has appeared or the first polar body has 

 already been separated off, it at once becomes evident that 

 important changes have occurred in the disposition and 

 relative proportions of the tAvo constituent regions of the egg- 

 cytoplasm. The full-grown ovum is of the centrolecithal 

 type, the central deutoplasmic zone forming its main bulk 

 and being completely surrounded by the thin formative zone. 

 The ripe ovum, on the other hand, exhibits an ol^vious and 

 unmistakable polarity, and is of the telolecithal type, as the 

 following facts show. The cytoplasmic body evidently con- 

 sists of the same two regions as form that of the full-grown 

 ovum, but here the dense formative region now forms its 

 main bulk, and no longer surrounds the clear deutoplasmic 

 region as a uniform peripheral layer. It has not only 

 increased considerably in amount as compared with that of 

 tlie full-grown eg'g, and at the expense apparently of the more- 

 peripheral coarser portion of the deutoplasmic zone, but it 

 has undergone polar segregation, with the result that it now 

 occupies rather more than one hemisphere of the egg as a 

 dense finely granular mass, with vacuoles of varying size 

 sparsely scattered througli it (figs, 2 and 3, f.z.). It 

 accordingly defines one of the ovular poles. The opposite 

 pole is just as markedly characterised by the presence imme- 

 diately below it of a more or less rounded clear mass, 

 eccentrically situated, and composed of an extremely fine 

 cytoplasmic reticulum with wide fluid-filled meshes. It is 

 completely surrounded by formative cytoplasm (though over 

 the polar region the enclosing layer is so extremely thin that 

 it here almost reaches the surface), and its cytoplasmic frame- 

 work is perfectly continuous with the same, the line of 

 junction of the two being abrupt and well defined. So 

 delicate, however, is this framework that it breaks down 

 more or less completely under the action of fixatives of such 



