The blood-corpuscles.—Blood-cells are said by Korotneff to be, in Gryllotalpa, at an early period present almost everywhere between the yolk and mesoderm; they are derived, as he states, from the cells of the somatic mesoderm layer, which has lost its connection with the other parts of the mesoderm, and fall into the body-cavity. Ayers states that the blood-corpuscles arise from serosa nuclei which have passed into the body-cavity, where they become more vesicular, and ultimately all of the nuclear substance goes to form from one to three spherical bodies, which are surrounded by the common membrane.
“These bodies are blood-corpuscles and are free nucleoli immediately on the rupturing of the vesicle which surrounds them.” (Ayers, Pl. 22, Figs. 1, 3, p. 250.) More recently, Schaeffer has observed in caterpillars certain cell-complexes associated with the fat-body which he has called blood-forming masses.
Musculature, connective tissue, fat-body.—The muscles of various parts of the body, as well as the connective tissue, arise by histological differentiation from the somatic layer of the mesoderm (Fig. 523, so). The fat-body originates from the same source, as shown by the researches of Kowalevsky, Grassi, and of Carrière. In Hydrophilus a dorsal band of the fat-body passes over the digestive canal arising by direct transformation of the wall of the cœlom-sacs. But also the other portions of the fat-body, as the fat-body lobes accompanying the tracheal system, are of undoubted mesodermal origin. Heymons’ observations on the cockroach (Phyllodromia) agree with the foregoing view. In this insect at a very early period certain cells in the wall of the cœlom-sacs undergo a change, and may be recognized as the rudiments of what are afterwards fat-body tissues (Fig. 540, B and C, f).
The reproductive organs.—Our knowledge of the mode of development of the genital organs is in a less satisfactory state than that of the other organs. It is now known that the rudiments of the sexual glands belong to the mesoderm, and are developed from the wall of the cœlom-sacs. In the cockroach (Phyllodromia), the most generalized of the winged insects, as Heymons has shown, in the earlier stages of the embryo separate genital cells are already distinguished by their histologically different characters from the other mesodermal cells. The genital cells are larger and show a feebly stained nucleus with a clear nucleolus. These genital cells, which are transformed normal mesodermal cells, lie originally within the mesoderm layer or on the surface of this layer turned towards the yolk, on the edge of the segments. After the complete formation of the cœlom-sacs we find them (Fig. 549, gz) in the dissepiments which separate the successive cœlom-sacs from one another. Here new genital cells are constantly formed through the transformation of mesoderm cells. The development of the genital cells takes place in the 2d to the 7th abdominal segments.
Afterwards the genital cells pass into the interior of the cœlom-sacs, and soon pass to the dorsal wall of the same (Fig. 540, A, gz) and enter between the cells of this wall. The cœlom-sacs (c) show in cross-section in this stage a triangular outline, so that we can distinguish a dorsal, lateral, and median wall. The dorsal wall lies next to the surface of the yolk, and afterwards gives rise by separation or splitting to the splanchnic mesoderm (Fig. 544, sp), while from its remains the terminal thread-plate (ef) originates. The lateral wall, which is turned towards the ectoderm of the primitive band, is intimately concerned in the formation of the somatic layer (Fig. 540, C, so) of the mesoderm. Out of what remains arises the pericardial septum (Fig. 544, ps).
When the genital cells have entered into the dorsal wall of the primitive segments, they are already so numerous that they form a continuous series extending from before backward. The genital rudiment consists, then, of a string of cells lying on each side in the dorsal wall of the primitive segments, which extend from the 2d to the 7th abdominal segments. In the formation of these strings or rows of cells not only are the genital cells concerned, but also still undifferentiated mesoderm cells (Fig. 540, B, C), which originate from the dorsal wall of the cœlom-sacs and lie next to the genital cells. Some of these last tend to envelop the genital cells. We designate them the epithelial cells of the genital rudiments (ep), while others form a cellular cord which takes a position medial and ventral to the genital cells.
Fig. 549.—Sagittal (longitudinal) section through the abdominal part of a primitive band of cockroach (Phyllodromia germanica) after the end of the formation of the primitive segments: 1–7, 1st to 7th abdominal segments; from the 8th abdominal segment (8) to the last segment (es) extends the inturned ventral part of the primitive band; am, amnion; c, cœlom-sac; d, yolk; gz, genital cells, lying partly in the dissepiments, partly in the wall or in the cavity of the primitive segments.
From the genital cells in the female arise only the egg-cells (and the nutritive cells in those forms which have such). The follicular epithelium of the egg-tube, on the other hand, also the corresponding cells of the terminal chamber, originate from the epithelial cells. Phyllodromia and Orthoptera in general, to which this description applies, show in this respect tolerably simple relations, since the germinal or terminal compartment of the ovary in them is composed of relatively few cells. In most other insects, and especially those which have a great number of food-cells in the ovary, the germinal chamber (Keimfach) is extraordinarily large.
The ventral cellular cord (cz) develops into the proximal part of the oviduct, which widens out and receives the single egg-tubes.