342 PALAEOSTRACA. 



blastodermic cuticle. This cuticle, which attains a considerable 

 thickness and, at later stages, when the chorion has been shed, is 

 the only covering of the embryo, is marked out into polygonal areas 

 which correspond to the limits of the individual blastoderm-cells 

 concerned in its secretion. 



Not all the cleavage-cells enter into the formation of the blasto- 

 derm. Many remain, filled with food-yolk, near the centre of tbe 

 egg. The sum total of these so-called yolk-cells represents the 

 entoderm. The blastoderm, on the other hand, contains the elements 

 of the future ectoderm and mesoderm.* 



The next processes, which must be described as gastrulation, and 

 which lead to the formation of the germ-band, now start from the 

 primitive cumulus. At the middle of this latter there first appears 

 a round depression, the blastopore, which soon becomes triangular 

 and then lengthens. A primitive groove thus forms (Fig. 155 A). 

 Simultaneously, behind the primitive cumulus, a second blastodermic 

 thickening arises, connected with the former; this, in superficial 

 figures, appears as a white patch. The primitive groove, which 

 lengthens posteriorly, soon stretches into this second prominence. 

 During this process the proliferation of mesoderm-cells takes place 

 from the primitive groove, these cells spreading out below the 

 ectoderm (Fig. 154). The groove here yields only mesoderm and 

 no entoderm. 



The extension of the mesoderm-elements below the ectoderm, 

 which proceeds from the primitive groove, appears in superficial 

 figures as a clear area surrounding the latter (Fig. 155 A). This 

 area soon extends, and the whole of the clear region under which 

 the mesoderm lies must be regarded as the commencing embryonic 

 rudiment, and may now be called the germ-disc. At its centre, 

 the primitive streak can be recognised, although this is much less 

 distinct in the stages which now follow than when it first appears. 



* [According to the latest researches of Kingsley (No. I.), each surface-cell 

 divides tangentially into a smaller external and a larger internal cell ; the latter 

 cells are rich in yolk and yield the entoderm, while the smaller cells yield the 

 mesoderm. In surface view the primitive cumulus appears like a pit, but this 

 is merely an optical effect produced by the greatly thickened centre. From this 

 point and from the primitive streak which extends backwards from it, we find a 

 proliferation of mesoderm-cells. 



KlSHINOUYB (No. II.) regards the ectoderm as having a double origin, that 

 of the ventral surface arising from the blastoderm, while that of the dorsal 

 surface arises from the yolk. The mesoderm, he states, arises in three ways \ 

 that of the cephalo-thorax from the deeper cells of the blastoderm, and thai of 

 the abdomen from the primitive streak; a third portion arises from the yolk-cells 

 and possibly gives rise to the blood-corpuscles.— Ed.] 



