Fig. 504.—Formation of the blastoderm of Pieris cratægi: A, longitudinal section through the egg, with two masses of protoplasm in the yolk. B, a blastoderm-cell at the upper end. C, a later stage, with more blastoderm-cells.—After Bobretsky.
The first step in segmentation is the movement of the first division-nucleus (i.e. that in the fertilized egg arising from the union of the sperm-nucleus with the female pronucleus) towards the interior of the egg in order to multiply itself by the mode of indirect nuclear division (Figs. 504, A, and 507).
Fig. 505.—Embryology of the mole-cricket: 1, egg in which the amœboid nuclei (abc) are moving toward the surface; 2, egg in which the nuclei (abc) have reached the surface, and show an active nucleus-formation; 3, the blastoderm-cells have no nucleus, and are placed at equal distances apart; 4, the blastoderm-cells now forming a continuous layer; 5, cross-section of the egg with blastodermic disk, also showing the disposition of the endodermal cells; 6, cross-section of the blastodermic disk, with the myoblast cells (mb) already formed; 7, cross-section through the thorax of the embryo, the body-cavity extended into the limbs.
Lettering.
abc, amœboid blastodermic cells. bc, blastoderm-cells. bl, blastoderm. en, endodermal cells. M″, cavity of the myoblast. mb, myoblast cells. N, nerve-furrow. P, primitive groove. pd, primitive disk.
Fig. 505 concluded.—Later stages in the embryology of the mole-cricket: 8, longitudinal section of the embryo; the yolk-pyramids (yp) form a common inner yolk-mass (y). 9, section through the heart; H, 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-lacunæ separated on the back by the dorsal organ (do); the intestinal fasciated layer (Darmfaserblatt) has not completely enclosed the yolk. 11, embryo completely segmented, with the rudiments of the appendages, labrum (lab), and nervous ganglia (pc-ng). 12, a more advanced embryo, showing the stomodæum (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.
Lettering.
ant, antenna. ar, arterial sinus. bl, blastoderm. bla, abdominal vesicles. cr, proventriculus, or crop. dm, ventral diaphragm. do, dorsal organ. d pm, dorsal diaphragm. ent, enteric layer. f, fat-body. g, ventral ganglion. H, ht, heart. l, lacuna. m, mouth. md, mandible. m.en, mesenteron. mx′, 1st maxilla. mx″, labium, or 2d maxilla. ml, leaf-like portion of mesenteron. oe, œsophagus. pc, procerebrum. pm, proctodæum. sg, subœsophageal ganglion. sm, stomodæum. tg, thoracic ganglion. vm, ventral muscle. y, yolk. yp, yolk-pyramids. I, 1st pair of feet. II, 2d pair of feet. III, 3d pair of feet.