The mesoblast arises in connection with the ventral thickening of the blastoderm, but the details of its formation are not known. Metschnikoff describes a longitudinal furrow which appears very early in Strongylosoma, which is perhaps equivalent to the mesoblastic furrows of Insects, and so connected with the formation of the mesoblast.
The mesoblast is divided up into a series of protovertebra-like bodies—the mesoblastic somites—the cavities of which become the body cavity and the walls the muscles and probably the heart. They are (Metschnikoff) prolonged into the legs, though the prolongations become subsequently segmented off from the main masses. The splanchnic mesoblast is, according to Metschnikoff, formed independently of the somites, but this point requires further observation.
The origin of the hypoblast remains uncertain, but it appears probable that it originates, in a large measure at least, from the yolk segments. In the Chilognatha the mesenteron is formed in the interior of the yolk segments, so that those yolk segments which are not employed in the formation of the alimentary canal lie freely in the body cavity. In the relation of the yolk segments to the alimentary canal the Chilopoda present a strong contrast to the Chilognatha, in that the greater part of the yolk lies within their mesenteron. The mesenteron is at first a closed sack, but is eventually placed in communication with the stomodæum and the proctodæum. The Malpighian bodies arise as outgrowths from the blind extremity of the latter.
Bibliography.
(397) G. Newport. “On the Organs of Reproduction and Development of the Myriapoda.” Philosophical Transactions, 1841.
(398) E. Metschnikoff. “Embryologie der doppeltfüssigen Myriapoden (Chilognatha).” Zeit. f. wiss. Zool., Vol. XXIV. 1874.
(399) —— “Embryologisches über Geophilus.” Zeit. f. wiss. Zool., Vol. XXV. 1875.
(400) Anton Stecker. “Die Anlage d. Keimblatter bei den Diplopoden.” Archiv f. mik. Anatomie, Bd. XIV. 1877.
Insecta[167].
The formation of the embryonic layers in Insects has not been followed out in detail in a large number of types; but, as in so many other instances, some of the most complete histories we have are due to Kowalevsky (No. [416]). The development of Hydrophilus has been worked out by him more fully than that of any other form, and will serve as a type for comparison with other forms.
Fig. 176. Four embryos of Hydrophilus piceus viewed from the ventral surface. (After Kowalevsky.)