OVIPOSITION, CLEAVAGE, AND FORMATION OP GERM-LAYERS. 341 



(No. 22), Brooks and Bruce (No. 10), and Kingsley (No. 15). 

 Judging from these, considerable agreement Avitli the ontogeny of 

 the Arachnida, especially the Scorpiones, appears to prevail. In 

 tracing the first stages of development we shall chiefly follow the 

 more recent observations of Kingsley. 



The first cleavage-nucleus lies, surrounded by formative yolk, 

 near the centre of the egg.*" Repeated division gives rise to a large 

 number of cleavage-nuclei which become distributed within the egg 

 before cleavage of the egg-contents (formation of definite cell-areas) 

 takes place. This distribution is not regular, for the cell-nuclei 

 wander to the surface most abundantly at the point where, in 

 later stages, the first rudiment of the embryo will appear. Since 

 a demarcation of the cleavage-cells takes place first at this point, 

 the cleavage is apparently meroblastic (discoidal) in character, 

 and thus recalls the processes observed in the Scorpiones (Vol. iii.. 

 Figs. 1 and 2). Finally, however, the whole egg breaks up into 

 segmentation -spheres which are chiefly composed of food-yolk 

 elements ; each, however, contains a cleavage-nucleus surrounded 

 by protoplasm. 



The Avandering to the surface of the cleavage-nuclei mentioned 

 above leads, by the demarcation of cells in that region, to the 

 formation of the blastoderm. The latter is thus first completely 

 formed at the special point referred to, and in these stages presents 

 the appearance of an 



accumulation of closely f) ^ 



packed cells at this pole, 

 while the rest of the 



surface of the egg is -x>A:iSi'js>*yi<^^-^-w 



still covered with large -n ^.a r^ <.- ^.v i <.v j- *• 



o Fig. 154. — Cross section through the germ -disc of 



cells filled with food- lAmulus showing the formatioii of the germ - layers 



11 -r, ,, , (after Kingsley). b, blastopore (primitive groove); 



yOlK. it appears tliat, ^^^ yolk-cells ; ec, ectoderm ; la, mesoderm. 



in later stages also, a 



blastodermic thickening is retained at this spot after the blastoderm- 

 formation has advanced over the rest of the surface ; this thickening 

 may be compared with the primitive cumulus (CLAPARfeoE) of the 

 spider's egg. 



While the formation of the blastoderm is still going on, a cuticular 

 secretion takes place at the surface of the blastoderm-cells, this 

 leading, as in many Crustacea (p. 118), to the development of the 



* [Kingsley surmises this, but was unable to find the first cleavage-nucleus. 

 —Ed.] 



