614 



ZOOLOGY 



SECT. 



Each of the two mesoderm bands undergoes transverse division 

 into a series of segments (somites), which become hollow and are 

 then closely applied to one another, eventually coalescing, so 

 that the cavities of all of them unite to form the coelome, the 

 outer walls becoming applied to the ectoderm to form a somatopleure, 

 or lamina consisting of somatic layer of mesoderm and of ectoderm ; 

 the inner being applied to the endoderm to form a splanchnopleure, 

 or lamina consisting of splanchnic layer of mesoderm and endoderm. 

 When the two rows of somites, right and left, become approximated 



FIG. 510. Ventral plate of embryo Cock- 

 roach (Blatta germanica), isolated 

 from the yolk. as. amnion and serosa ; at. 

 antennary lobe ; a/I. brain ; cpl. caudal 

 plate ; Ib. labrum ; md. mandible ; mx 1 , 

 mx 2 , first and second maxillae ; p 1 , p%. p : \ 

 legs. (After Wheeler.) 



FIG. 511. Embryo Cockroach just after 

 the rupture of the amnion and serosa, 

 lateral view of entire egg. Letters as in 

 preceding figure. In addition, at. fatty 

 body ; ast. caudal styles b. cephalic end of 

 yolk ; oc. eye. (After Wheeler.) 



dorsally, special cells the cardioblasts separated off from them 

 combine to form the wall of the tubular heart. Long before this 

 is formed, however, the sides of the embryo are observed to undergo 

 regular pulsations. The original coelome the space between the 

 somatopleure and splanchnopleure has become converted into, 

 or replaced by, a blood-sinus (hsemocoele), divided up into a number 

 of smaller lacunae containing plasma by films of connective tissue. 

 The ventral plate gradually grows upwards at the sides, and 



