"266 INSECT A. 



cylindrical epithelium which continuously covers the surface of 

 the egg. 



The point at which the cleavage-cells first reach the surface varies in the 

 different groups of Insects. In the Muscidae, according to Graber, the formation 

 of the blastoderm first commences at the posterior pole of the egg, while in Apis 

 (Kowalevsky), Pieris (Bobretzky, No. 6), and Chironomus (Weismann, 

 No. 89), the first blastoderm-cells were noticed at the anterior pole. In Hydro- 

 philus (Heidek, No. 38) the blastoderm first forms round the middle of the egg 

 as a transverse girdle, somewhat nearer the posterior pole of the egg, and develops 

 last at the poles. In Blatta (Wheeler) and Gryllotalpa (Korotneff) the first 

 cells forming the blastoderm appear on the future ventral surface. As it is at 

 this side that the rudiment of the germ-band arises, the early appearance of the 

 blastoderm-cells at this part recalls the premature development of the blastoderm 

 often occurring in the Crustacea in the region of the embryonic germ-zone 

 (Vol, ii., p. 115). A similar development is found in Occanthus (Ayers, No. 1). 



A method of blastoderm-formation differing somewhat from the 

 above and more normal type has been observed in some Orthoptera 

 (Blatta and Gryllotalpa). As a rule, the "cleavage-cells" increase 

 within the food-yolk so rapidly that when they reach the surface 

 of the egg they are closely crowded together, and here at once 

 constitute a continuous epithelium, but this is not the case in 

 Gryllotalpa (Weismann, No. 89, and Korotneff, No. 47) and 

 in Blatta (Wheeler, No. 95). In these forms the first "cleavage- 

 cells," which are comparatively few in number,* migrate to the 

 ventral surface of the egg and there multiply so that separate cell- 

 islands are temporarily formed. Only in later stages do the cleavage- 

 cells, greatly increased in number by division, become distributed 

 equally over the whole surface of the egg. It was maintained by 

 Wheeler that, in Blatta, when the amoeboid cleavage-cells had 

 reached the surface of the yolk, their nuclei no longer showed 

 mitotic division, but here (as well as later in the serosa) multiplied 

 by direct or amitotic division. 



The question as to the origin of the so-called yolk-cells or vitello- 

 phags is of importance. It has been observed that, as a rule, not 

 all the " cleavage-cells " shift to the surface to take part in the 

 formation of the blastoderm, but that a few remain behind within 

 the yolk (Fig. 130 D, cte, and 131 C, D, z), where they increase in 

 number, obtain equal distribution throughout the yolk, and become 

 the so-called yolk-cells, whose function is to liquefy the mass of 

 food-yolk and to bring about its assimilation. The origin of the 

 yolk-cells from cleavage-cells which have remained in the yolk has 



* [In Blatta they are numerous, sixty to eighty cells being found scattered 

 in the yolk before migration commences. — Ed.] 



