Chap. XIII.] THE COCKROACH. 277 



mid-line as does the vertebrate amnion fold, forms a double 

 investment of the plate, an inner true amnion and an outer serous 

 membrane. As the ventral plate increases in size and tends to 

 enclose the embryo, the point of origin of these membranes 

 is forced to retreat dorsally, until at last, when the plate 

 envelopes the whole embryo, the amnion and the serous mem- 

 brane become separated off, and form complete membranous 

 investments of the embryo. 



The heart is mesoblastic in origin. When the cavities of the 

 lateral mesoblastic somites coalesce to form the coelom, the inner 

 wall becomes the splanchnic layer, the outer wall the somatic 

 layer. At the edges of the mesoblastic bands the layer of cells 

 which unites the somatic with the splanchnic wall forms, on 

 each side, a grooved cardiac rudiment. As the hollowed meso- 

 blastic bands grow upwards so as to approach the dorsal surface, 

 these grooved rudiments tend to meet ; and the tubular heart 

 arises by their ultimate coalescence. 



The trachea! system arises from tubular infoldings of the 

 epiblast. Each stigma has at first a distinct system, but these 

 coalesce into a common system. 



The reproductive organs are of mesoblastic origin. 



