220 



MYRIOPODA. 



1. Cleavage and Formation of the Germ-layers. 



The cleavage of the Myriopodan egg has repeatedly been described 

 as total, but deserves this designation even less than, for example, 

 the eggs of the Araneae. The egg, it is true, shows a number of 

 segments, which at first is not large, but increases later, and these 

 produce the appearance of division into more or less sharply marked 

 blastomeres, but this appearance is not the expression of total 

 cleavage in the strict meaning of the term, and only arises some 

 time after the division of the cleavage-nucleus and its descendants 

 inside the egg. 



Fig. 107.— Sections through eggs of Geophilus ferrutjincus showing the blastoderm-formation 

 (after Sografk). hi, blastoderm ; dp, yolk-pyramids ; gr, groups of blastoderm-cells on tin- 

 future dorsal surface ; k, nuclei with the protoplasmic areas surrounding them. 



The cleavage of the external surface of the egg does not appear to take place 

 in all Myriopodan eggs. Heathcote, for instance, points out specially, in 

 connection with the Julus tcrrestris, Leach, that no outward segmentation 

 is to be observed in this form, although Metschnikoff, in another species 

 of Julus (J. Morelletti, Lucas), carefully described and figured the segmentation 

 of the surface of the egg. The absence of this external cleavage in other 

 species is perhaps to be accounted for by the great abundance of yolk in these 

 forms. 



The cleavage-nucleus lies, surrounded by a mass of protoplasm, in 

 the centre of the egg. It here divides first into two, and these soon 

 increase by further division, so that many nuclei, each surrounded 

 with an area of protoplasm, are found at this stage in the centre of 



