VIII 



ARTHEOPODA 



247 



primitiye cumulus iu an Arachnid egg, and is the first rudiment of 

 the genital organs. We arrive, then, at a stage when the egg is com- 

 pletely surrounded by a layer of cells, a blastoderm in fact, and when it 

 contains m its anterior a considerable number of isolated nuclei, which 

 become surrounded by cytoplasm and form the so-called yolk cells. 



In Boryphora, according to "Wheeler, the yolk subsequently 

 segments into a number of spherical masses, each containing two or 

 three nuclei and each surrounded by a thin layer of cytoplasm, and 

 Wheeler regards these masses as really large 

 yolky cells. Hirschler does not describe this 

 process in Donacia. 



The next change which occurs is a peculiar 

 invagination of a portion of the dorsal blasto- 

 derm. At first this looks like a groove which 

 is overgrown from the sides by the adjacent 

 blastoderm, and it finally spreads out as a sheet 

 beneath the surface of the ectoderm. This sheet 

 degenerates and disappears; it is regarded as the 

 " primary dorsal organ," because, as we shall 

 see, similar processes occur at a much later stage 

 in development, and this later infolding struc- 

 ture is called the secondary dorsal organ. 



Fig. 194. — Surface view of 

 the egg of Donacia cras- 

 sipes at the conclusion 

 of blastoderm formation. 

 (After Hirschler.) 



germ, primitive germ cells ; 

 ser, cells destined to form the 

 serosa. 



Fig. 195. — Section through the dorsal part of a 

 developing egg of Donacia crassipes to show 

 the primitive dorsal organ. (After Hirschler.) 



ect, blastodermic ectoderm ; p. do, primitive dorsal 

 organ. 



As soon as this primary infolding has taken place, the dorsal 

 blastoderm in front of it begins to exhibit a different character from 

 the blastoderm elsewhere ; its nuclei become larger and much more 

 widely spaced than the nuclei elsewhere (ser, Fig. 194). This peculiar 

 ectoderm forms a V-shaped area, with the point directed backwards 

 and the broad end forwards. It soon attains the anterior pole of the 

 egg and is the rudiment, as appears later, of the outer embryonic 

 membrane, the serosa. The serosa protects the embryonic area of 

 the egg during its development, and for this reason is termed the 

 sheath-ectoderm. It is probable that this peculiar change indicates 

 a change in physiological function; it is suggested that the serosa 

 ectoderm is specially suited to promote gaseous interchange between 

 the egg and the surrounding medium. 



