CLEAVAGE AND FORMATION OF THE BLASTODERM. 



107 



observations agree with those of Weismann, who watched the passage of the 

 naked egg into the brood-cavity in various Cladocera, and the subsequent forma- 

 tion of the membrane. More recently, Bella Valle (No. 76) has shown that 

 in Gammarus also the eggs are passed on into the brood-cavity without an 

 external envelope, and only secrete the vitelline membrane 'after fertilisation. 



Secondary external envelopes are often present in addition to the 

 vitelline membrane. Among these must be enumerated the external 

 hard shell of the winter eggs of the Phyllopoda (Fig. 50, d, p. 108), 

 the ovisacs of the Copepoda and the Cirripedia, and the membrane 

 of attachment (stalked egg-shell) of the Decapoda, which does not 

 always completely surround the egg. 



There is much variation in the cleavage of the egg in different 

 forms of the Crustacea, but these various types of cleavage cannot 

 be assigned with any exactitude to the different subdivisions of the 

 group, since distinct kinds of cleavage are to be found in nearly 



Pig. 49.— Three stages in the cleavage of the egg of Lucifer (after Brooks). A, stage- showing 

 division into eight cells. B, blastnla stage with central cleavage-cavity. C, gastrula stage. 

 d, yolk-containing portions arising from the cell c. 



related forms. Gammarus affords an example of this, the different 

 species of this genus showing variations of cleavage which, however, 

 according to Della Yalle (No. 76), are not so remarkable as we were 

 led to believe by the earlier investigations of La Valette St. George 

 (No. 77), Van Beneden (No. 1), and Bessels (No. 2). Similar 

 examples might be cited from among the parasitic Copepoda and the 

 Cladocera. The latter group exhibits particularly clearly how the 

 type of cleavage is influenced by the quantity of food-yolk present, 

 and by the possibility of the egg being otherwise provided with 

 nutritive material. In many forms of this sub-order, the winter egg, 

 which is rich in food-yolk, differs in the type of its segmentation 

 from the summer egg, which is poor in yolk, and which, during the 

 whole course of its embryonic development, receives from the mother 

 fluid nourishment through the albuminiferous contents of the brood- 

 cavity (Weismann, Claus). 



