Germ-Cells in Chrysomelicl Beetles. 2G5 



(Fig. 23) the blastoderm ends abruptly at the place where it meets 

 the pole-cells; in the older stage (Fig. 24) it has apparently pushed 

 past the latter on all sides, and now projects obliquely upward into 

 the yolk. A still later stage (Fig. 25) shows that these projecting 

 ends of the blastoderm finally meet, thus forming a structure which 

 in longitudinal section appears as an inverted V, but in reality is 

 a cone-shaped ping extending into the yolk. At first the nuclei 

 are arranged near the surface of this cone in a fairly regular row; 

 some of them, however, soon become displawd. By the time the 

 genesis of the pole-cells is completed, these nuclei form an irregular 

 group, just within the egg opposite the pole-cells with which they 

 are connected by a mass of cytoplasm. We shall see later that they 

 remain thus in communication with the pole-cells for a long period 

 of embryonic growth. In toto preparations show this group lying 

 within the yolk in the above described position (Fig. 16, ps. hi. n.). 



3. Tlie History of the Pole-Cells Until the Sex of the Embryo 

 Can he Recognized. 



In order to follow the history of the pole-cells, it is necessary 

 to describe the development of the germ-band. The blastoderm, as 

 stated in the last chapter, is present at the conclusion of the pole- 

 cell formation, as a single layer of regularly arranged cells covering 

 the entire surface of the egg, except a small area at the posterior end. 

 The first change noticed in the blastoderm is a crowding together of 

 the cells on the ventral surface of the egg. This results in the forma- 

 tion of a broad longitudinal band of closely aggregated cells, the 

 ventral plate (Stage C). The edges of this plate are thrown up into 

 two folds ; these spread out in the posterior region extending to the 

 end of the egg (Stage D), where they pass around the pole-cells 

 and meet on the dorsal surface (Fig. 31). The entire ventral plate 

 now decreases both in length and in breadth ; during this contraction 

 a longitudinal concavit}'', the ventral groove, appears (Stage E). 

 By this shortening of the germ-band, the pole-cells are carried from 

 their former position at the end of the egg (Stage B) to a point 

 slightly anterior, on the ventral surface (Stage E) ; bore they occupy 



