xi PHYLUM ARTHROPODA 613 



distributed through a considerable portion of the yolk. These 

 (Fig. 509) all migrate to the surface, where they multiply rapidly 

 and form a layer, the blastoderm, which becomes thickened along 

 the ventral surface by the cells being elongated in a vertical 

 direction. From the blastoderm a number of cells pass inwards 

 into the substance of the yolk, where their function is to convert 

 the yolk-material into various soluble substances for the nourish- 

 ment of the blastoderm. The ventral thickening of the latter is 

 the ventral plate : its cells proliferate, and the plate comes to be 

 several cells thick : in front it becomes broader an indication of 

 the position of the future head-lobes. At the opposite end there is 

 a specially thickened area of the ventral plate with a slight depres- 

 sion on its surface ; the depression perhaps represents the blastopore, 

 since it is from this point forwards that the formation of the meso- 

 derm proceeds. The latter is formed as a longitudinal band which 

 bifurcates in front in the position of the head-lobes. 



The mode of origin of the endoderm in the Cockroach is not 

 known with certainty. It appears beneath the mesoderm, in two 

 separate portions, as a thin layer of cells one portion, the anterior, 

 coming into relation with the beginnings of the stomodaeum, which 

 arises as an invagination from the surface in the region of the head- 

 lobes, and the other, the posterior, uniting with the proctodaeum, 

 a similar ectodermal invagination at the posterior end of the ventral 

 plate. These two rudiments of the endoderm grow towards one 

 another, and eventually meet to form a continuous layer destined 

 to form the wall of the mesenteron. The ventral plate early becomes 

 divided by a number of narrow transverse lines which indicate the 

 boundaries of the future segments. 



Kudiments of appendages (Figs. 510, 511) appear on the head 

 and thorax, and a series also appears on the abdomen ; all of the 

 latter, however, subsequently disappear with the exception of the 

 last j^air, which give rise to the cerci. The segment on which the 

 rudiments of the antennae appear is at first post-oral in position, 

 but subsequently becomes fused with a pre-oral segment (pros- 

 tomium), so that the antennae acquire their permanent pre-oral 

 position only secondarily. The prostomial segment, the antennary 

 segment, a segment devoid of appendages, the segment bearing 

 the rudimentary mandibles, and those bearing the two pairs of 

 maxillae six segments in all unite to form the head of the adult. 



Then follows the appearance of the larval membranes. On 

 either side arises a fold of the blastoderm ; and the two folds 

 grow inwards and eventually unite over the body of the embryo, 

 forming a complete two-layered covering for it. The outer layer 

 is termed serosa, the inner amnion. 1 



1 This term is derived from one used in the Vertebrata, in which there is 

 an analogous membrane, occupying, however, a dorsal instead of a ventral 

 position as regards the body of the embryo. 



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