FORMATION OF THE BLASTODERM 



529 



The origin of numerous division-nuclei as the offspring of the first has been 

 observed to take place in the eggs of those insects (Aphides, Cecidomyia, and 

 Cynips) which have a slight amount of yolk. Yet in the large, ordinary eggs of 

 insects with an abundance of yolk there is no doubt, say Korschelt and Heider, 

 that these numerous division-nuclei, which soon after the process of oviposition 

 are scattered within the egg between the yolk-spheres, and are enveloped by a 

 star-shaped protoplasmic layer, and which constitute the formative elements 

 of the blastoderm, there is no doubt but that they have practically arisen 

 through indirect nuclear division from the first division-nucleus. 



The process of formation of the blastoderm in ordinary eggs with abundant 

 yolk was first observed by Bobretsky in the eggs of a moth (Porthesia) and 

 Pieris, also by Graber, and more recently by Blochmann in Musca, and by 

 Heider in Hydrophilus. 



In the earliest stage observed by Bobretsky there first appear after fertiliza- 

 tion a few (the smallest number four) cell-like, minute amoeboid masses of 

 protoplasm, each with a distinct nucleus. A few (one at least) of these bodies 

 gradually pass out of the centre of the yolk to the surface of the egg (Fig. 504, 

 A, n), these becoming larger and rounder, and from one or two of these nuclei 

 (B, be) the blastoderm originates (<7, bl). Those nuclei remaining in the yolk 

 increase in number and afterwards become the nuclei of rounded masses of yolk- 

 granules, forming the so-called yolk-spheres which Bobretsky regards as true cells. 



To the few blastoderm cells situated on the upper end of the egg are added 

 others which continue to pass from the yolk to the periphery, and then the 

 blastoderm spreads out farther and farther from the upper end of the egg until 

 finally it covers or envelops the whole yolk. This layer of cells is called the 

 blastoderm. 



As to the origin of the primitive amoeboid cells, Bobretsky is in doubt, but is 

 disposed to think that they are the result of the subdivision of the germinative 

 vesicle or nucleus of the ovarian egg-cell. In this connection may be quoted the 

 observations of Graber, who states that an examination of the ovarian cell at an 

 early period has revealed the presence, in the centre of the yolk, of a number of 

 amceboid cells, which appear to have been formed by the division of the germi- 

 nal vesicle. These " primary embryonic cells " have a relatively large nucleus 

 and a number of nucleoli. Several may be seen to unite with one another by 

 means of their pseudopodia, and they may also be observed to undergo division. 

 With this account may be compared the results obtained by Korotneff in his 

 work on the embryology of the mole-cricket (Fig. 505). 



FIG. 505 concluded. Later stages in the embryology of the mole-cricket: 8, longitudinal sec- 

 tion of the embryo ; the yolk-pyramids (yp) form a common inner yolk-mass (y). 9, section through 

 the heart; //, cavity of the heart; the two halves of the heart-sinuses having united dorsally,.ventrally 

 they are still open and are bounded by the walls of the mesenteron. 10, cross-section of an embryo, 

 showing the blood-lacunae separated on the back by the dorsal organ (do) ; the intestinal ftisciated layer 

 (Darmfaserblatt) has not completely enclosed the yolk. 11, embryo completely segmented, with the 

 rudiments of the appendages, labruin (lab), and nervous ganglia (pc-ng). "12, a more advanced 

 embryo, showing the stomodieum (st) indicated as a frontal protuberance. 13, section through the 

 recently hatched larva, showing the cells of the mesenteron or chyle-stomach, and the cellular layer 

 on the front surface, also the proventriculus or crop. 



ant, 



ar, 



bl, 



bla, 



cr, 



dm, 



do, 



dpm, 



ent, 



f, 



Q, 



antenna, 

 arterial sinus, 

 blastoderm, 

 abdominal vesicles, 

 proventriculus, or crop, 

 ventral diaphragm, 

 dorsal organ, 

 dorsal diaphragm, 

 enteric layer. 

 fat-body, 

 ventral ganglion. 



2M 



LETTERING. 



//, M, heart. 

 I, lacuna. 

 m, mouth. 

 md, mandible. 

 m.en, mesenteron. 

 mx', 1st maxilla. 

 mx", labium, or 2d maxilla. 

 ml, leaf-like portion of me- 

 senteron. 

 oe, oesophagus. 

 pc, procerebrum. 



pm, proctodwum. 



xff, suboesophageal ganglion. 



sin, stoiuodanim. 



tff, thoracic ganglion. 



i-m, ventral muscle. 



y, yolk. 



yp, yolk-pyrarnids. 



/, 1st pair of feet. 



//, 2d pair of feet. 



7/7, 3d pair of feet. 



After Korotneff. 



