2 TUE DEVELOPMENT OF 



gcneni Theridion, Pholciis, Drassus and Lycosa, — a paper 

 the poiuts of "wbich appear to fully support the views which 

 were advaiiced in the discussion. In brief, according to 

 Morin,the nucleus of segmentation in the eggs stndied by 

 himlies at the centre of the egg and not nntil the third seg- 

 mentation does the yolk segment. The segmentation is 

 perfectly regulär and not nntil the one hundred and twen- 

 ty-eight-cell stage is reached, do the nuclei and surround- 

 ing protoplasm reach the surface and form the blastoderm, 

 at which time they separate from the yolk pyramids, which 

 nowagain forms an nnnucleated homogeneous mass. The 

 blastoderm now thickens upon the ventral surface, and 

 from its centre cells are budded inwards, some of which re- 

 main between the parent cells and the yolk, while others 

 sink into the yolk itself. As will readily be seen this pro- 

 cess, which gives rise to both meso- and entoderm, is easily 

 brought into füll accord with the gastrulation in Crangon, 

 and other types. Morin does not regard the " primitive 

 cumulus " as of importance in the formation of the germ 

 layers, since, in bis experience, it does not arise until after 

 they are formed ; indeed it does not appear at a^iy stage in 

 Theridion. The entoderm nuclei sink into the yolk where 

 they remain distinct — much as I have described them in 

 Limulus ('85, p. 530, ligs. 45 and 47) —until shortly be- 

 fore hatching. 



Schimkewitsch, who has also published bis complete 

 paper ('87) on the development of Arachnida, differs con- 

 siderably from Morin in bis interpretations. He thinks 

 that only a portion of the products of segmentation migrates 

 to the surface to form the blastoderm, the others remaining 

 behind to form polynuclear yolk pyramids (not seen by 

 Morin) which represent the entoderm. The early appear- 

 ance of the primitive cunuilus and the white spot are in- 

 terpreted by Schimkewitsch as indicating the limit of meso- 



