(439) M. Balbiani. “Mémoire sur le développement des Aranéides.” Ann. Scien. Nat. Series V. Vol. XVII. 1873.
(440) F. M. Balfour. “Notes on the development of the Araneina.” Quart. Journ. of Micr. Science, Vol. XX. 1880.
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(442) E. Claparède. Recherches s. l’évolution des Araignées. Utrecht, 1862.
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Acarina.
(445) P. van Beneden. “Développement de l’Atax ypsilophora.” Acad. Bruxelles, t. XXIV.
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Formation of the layers and the embryonic envelopes in the Tracheata.
There is a striking constancy in the mode of formation of the layers throughout the group. In the first place the hypoblast is not formed by a process which can be reduced to invagination: in other words, there is no gastrula stage.
Efforts have been made to shew that the mesoblastic groove of Insects implies a modified gastrula, but since it is the essence of a gastrula that it should directly or indirectly give rise to the archenteron, the groove in question cannot fall under this category. Although the mesoblastic groove of Insects is not a gastrula, it is quite possible that it is the rudiment of a blastopore, the gastrula corresponding to which has now vanished from the development. It would thus be analogous to the primitive streak of Vertebrates[184].
The growth of the blastoderm over the yolk in Scorpions admits no doubt of being regarded as an epibolic gastrula. The blastopore would however be situated dorsally, a position which it does not occupy in any gastrula type so far dealt with. This fact, coupled with the consideration that the partial segmentation of Scorpio can be derived without difficulty from the ordinary Arachnidan type (vide p. [120]), seems to shew that there is no true epibolic invagination in the development of Scorpio.
On the formation of the blastoderm traces of two embryonic layers are established. The blastoderm itself is essentially the epiblast, while the central yolk is the hypoblast. The formation of the embryo commences in connection with a thickening of the blastoderm, known as the ventral plate. The mesoblast is formed as an unpaired plate split off from the epiblast of the ventral plate. This process takes place in at any rate two ways. In Insects a groove is formed, which becomes constricted off to form the mesoblastic plate: in Spiders there is a keel-like thickening of the blastoderm, which takes the place of the groove.
The unpaired mesoblastic plate becomes in all forms very soon divided into two mesoblastic bands.
The mesoblastic bands are very similar to, and probably homologous with, those of Chætopoda; but the different modes by which they arise in these two groups are very striking, and probably indicate that profound modifications have taken place in the early development of the Tracheata. In the Chætopoda the bands are from the first widely separated, and gradually approach each other ventrally, though without meeting. In the Tracheata they arise from the division of an unpaired ventral plate.