236 Eobert Willielin Ilejnier, 



b'- 



several layers thick ; here the inner cells were large, richer in yolk, 

 and their nuclei had one or two nucleoli, but less chromatin than the 

 (overlying) blastoderm cells. These inner, primitive genn-cells soon 

 became amoeboid, and, several hours before mesoderm formation, 

 separated into groups ^^'hieh moved forward through the yolk. Each 

 group divided, half of the cells migrating to either side of the body, 

 where they lay near the coelom in the fourth to sixth abdominal 

 segments. Their history was not carried further. 



2. Diptera. 



The Diptera have received their full share of attention from 

 insect embryologists and the works relating to species of this order 

 are very numerous. They are also the first, and for a long time the 

 only insects in whose eggs pole-cells were discovered; this group is, 

 therefore, of great interest to us since pole-cells are also present in 

 the development of the Chrysomelid beetles considered in this paper. 



Robin (1862) unknowingly discovered the early segregation of 

 germ-cells from somatic cells in the nearly transparent eggs of 

 Tipulides culiciformes. Before the blastoderm Avas formed there 

 appeared "par gemmation de la substance hyaline du vitellus" a 

 number of "globules polaires" which he likened to the polar bodies 

 of other animals. These four to eight buds each developed a nucleus 

 which gave to it the character of a true cell. These cells Avere sup- 

 posed to take part in the formation of the blastoderm near the spot 

 where they were protruded. Robin did not orient the eggs correctly 

 for he says "C'est aussi le point oii apparaitre plus tard I'extremite 

 cephalique," a statement that Weismann corrected the following 

 year. 



Although Weismann (1863) had observed these "globules polaires" 

 in the eggs of Gliironomus nigro-viridis and Musca vomitoria several 

 years l^efore Robin's papers appeared, he did not publish his results 

 until 1863. Weismann noticed at the pointed posterior end of the 

 egg four indefinite, bright spots lying in the "Keimhautblastem." 

 These developed into four bud-like protrusions which were entirely 

 cut off from the egg and lay in the space between the vitelline mem- 

 brane and the surface of the egg. These "globules polaires" or "Pol- 



