do. dorsal plate; vn. ventral nerve cord; al. mesenteron; ht. heart.
The large spaces at the sides are parts of the body cavity.
In the Poduridæ the embryonic membranes appear to be at any rate imperfect. Metschnikoff states in his paper on Geophilus that in some ants no true embryonic membranes are found, but merely scattered cells which take their place. In the Ichneumonidæ the existence of two embryonic membranes is very doubtful.
Formation of the embryonic layers. The formation of the layers has been studied in sections by Kowalevsky (No. [416]), Hatschek (No. [414]), and Graber (No. [412]), etc. From their researches it would appear that the formation of the mesoblast always takes place in a manner closely resembling that in Hydrophilus. The essential features of the process ([figs. 177] and [178]) appear to be that a groove is formed along the median line of the ventral plate, and that the sides of this groove either (1) simply close over like the walls of the medullary groove in Vertebrates, and so convert the groove into a tube, which soon becomes solid and forms a mass or plate of cells internal to the epiblast; or (2) that the cells on each side of the groove grow over it and meet in the middle line, forming a layer external to the cells which lined the groove. The former of these processes is the most usual; and in the Muscidæ the dimensions of the groove are very considerable (Graber, No. [411]). In both cases the process is fundamentally the same, and causes the ventral plate to become divided into two layers[171]. The external layer or epiblast is an uniform sheet forming the main part of the ventral plate ([fig. 178] B, ep). It is continuous at its edge with the amnion. The inner layer or mesoblast constitutes an independent plate of cells internal to the epiblast ([fig. 178] B, me). The mesoblast soon becomes divided into two lateral bands.
The origin of the hypoblast is still in dispute. It will be remembered (vide pp. [114] and [116]) that after the segmentation a number of nuclei remain in the yolk; and that eventually a secondary segmentation of the yolk takes place around these nuclei, and gives rise to a mass of yolk cells, which fill up the interior of the embryo. These cells are diagrammatically shewn in [figs. 181] and [189], and it is probable that they constitute the true hypoblast. Their further history is given below.
Formation of the organs and their relation to the germinal layers.
The segments and appendages. One of the earliest phenomena in the development is the appearance of transverse lines indicating segmentation ([fig. 186]). The transverse lines are apparently caused by shallow superficial grooves, and also in many cases by the division of the mesoblastic bands into separate somites. The most anterior line marks off a præ-oral segment, which soon sends out two lateral wings—the procephalic lobes. The remaining segments are at first fairly uniform. Their number does not, however, appear to be very constant. So far as is known they never exceed seventeen, and this number is probably the typical one ([figs. 186] and [187]).
In Diptera the number appears to be usually fifteen though it may be only fourteen. In Lepidoptera and in Apis there appear to be sixteen segments. These and other variations affect only the number of the segments which form the abdomen of the adult.
