I08 CLEAVAGE AND DIFFERENTIATION 



the former period portions of cytoplasm are removed from the egg, 

 some of the costae may be entirely absent ; if, on the other hand, 

 portions (even quite large) of cytoplasm are removed from the egg 

 in the latter period (which, it may be noted, is also later in time), 

 none of the costae are absent, although they may be small. ^ 



Another interesting example in which viscosity plays an im- 

 portant part is provided by the Ascidian egg. The unfertilised egg 

 is very fluid, and, indeed, as will be seen below (p. 119), extensive 

 internal rearrangements of the contents take place at fertilisation. 

 But 10 minutes after fertilisation, the cytoplasm takes on a high 

 degree of viscosity ; this is reduced for a short period at 40 minutes 

 after fertilisation, and then rises again.^ 



§4 

 In Beroe, in addition to a variable high viscosity, we find, as men- 

 tioned above, the precocious formation, prior to fertilisation, of 

 certain specific substances, which are apparently of an "organ- 

 forming " nature. As we shall see, precocious chemo-differentiation 

 of such substances is universal among so-called mosaic-eggs. As 

 a result of this precocity in their formation, the specific organ- 

 forming or morphogenetic substances are already formed in the 

 just-fertilised egg, instead of being produced only after gastrulation 

 as in Amphibia. If these morphogenetic substances are distributed 

 unevenly during cleavage, mosaic development is the result. One 

 of the classical illustrations of this is the Mollusc Dentalium. 



Dentalium is an example of that group of animals which exhibit 

 the remarkable form of determinate segmentation known as spiral 

 cleavage, to be found in most Molluscs and many worms. It will 

 be advisable to give a brief general description of this type of cleav- 

 age before continuing our discussion of Dentalium. The first two 

 cleavages are meridional, and are often unequal, so that one of the 

 cells at the 4-cell stage (blastomere D) is larger than the other three 

 {A, B, C). The next cleavage is latitudinal but very unequal,^ 



1 Yatsu, 1912A, b; Spek, 1926. ' " Dalcq, 1932. 



3 The inequality which characterises these cleavage divisions seems to depend 

 on a gradient of permeability extending through the cytoplasm of the dividing 

 cell. Ultra-violet ravs and MgCL render the permeability of the cytoplasm 

 uniform throughout the cell, and after exposure to these agencies cleavage divi- 

 sions (of the Lamellibranch Molluscs), which would normally be unequal, take 

 place equally (Pasteels, 193 1). 



