THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 25 



Balfour says the cumulus appears after the blastoderm has been full}' formed. This 

 is not true of the species of spider studied by me. The primitive cumulus con- 

 sists of a mass of undifferentiated cells, extending well into the centre of the egg. 

 (Fig. LXXV.) Their histological structure is well preserved in the section which 

 figure LXXV represents. The cells have large granular nuclei. The protoplasm 

 is marked by granular radii extending from the nucleus to the periphery of the cell. 

 These cells are primitive and unspecialized. They resemble the yolk cells which 

 are certainly like the cells occurring in the early stages of segmentation before 

 specialization of tissue has taken place. It is not important to enquire whether the 

 cumulus results from the division of surface cells or is formed by the accession of 

 cells from the yolk. It is very probably formed both by the division of cells which 

 have reached the surface, and by the addition to these of yolk cells. It is important 

 to note, however, that it consists of undifferentiated cells, and is formed, in the 

 species of spider which I studied, before the blastoderm is completed. In Limulus 

 the first trace of the embryo is also a mass of undifferentiated cells lying at the sur- 

 face of the egg. (1) 



Figure LXXVI represents the next stage obtained after the cumulus stage. It 

 is a longitudinal section showing nine or ten mesoblastic somites which are not 

 hollow at this stage. 



It seems that the cumulus must take part in the formation of some if not all of 

 these somites. 



In Limulus the mesoderm arises in part at least from the inner cells of the 

 cumulus. 



Balfour states that the somatic mesoderm arises from the ectoderm and the 

 splanchnic mesoderm from the yolk cells. In the early stages, however, the meso- 

 derm is not divided into two layers. Figure LXXVII represents yolk cells near the 

 surface undergoing what appears to be endogenous division. This corresponds to 

 an endogenous division observed by Reichenbach as taking place in the yolk cells of 

 Astacus. 



Figure LXXVIII represents a longitudinal section of an embryo more advanced 

 than that represented by figure LXXVII. Here the six thoracic and four provisional 

 abdominal appendages have appeared. 



ID University Circulars, 1885. 



