254 



INVERTEBEATA 



CHAP. 



emug. 

 covering 



daeum. The stomodaeal tube projects backwards and the proctodaeal 

 tube forwards. As they grow in length they indent the layer 

 of eudoderni cells on which they impinge, and so their inner ends 

 become clothed, so to speak, with a layer of eudoderni cells. In the 

 case of the stomodaeum this layer is continuous with the anterior 

 endodermic thickening underneath the stomodaeum, while in its 



passage backwards the stomo- 

 daeum passes over it. The 

 proctodaeal tube in its growth 

 forwards similarly passes over 

 the posterior endodermic thick - 

 The endodermic cells 

 the inner blind ends 

 of stomodaeum and procto- 

 daeuni, multiply rapidly and 

 give rise to two lateral streaks 

 of endoderm, right and left, in 

 front and behind, which extend 

 along the sides of the yolk. 



Meanwhile the middle sec- 

 tion of the original median 

 band of endoderm. has broken 

 up into a mass of rounded cells 

 situated in a space which be- 

 comes the median section of the 

 epineural sinus. This, as we 

 have seen, is one of the first 

 blood spaces to be differentiated 

 in Peripatus, it derives its name 

 from the circumstance that 

 it lies above the rudiments of 

 the ganglia of the ventral nerve 

 cord. The newly established 

 lateral bands of endoderm grow 

 towards the middle part of the 

 embryo and here meet, so that 

 the yolk, throughout its whole 

 length, is covered with a layer 

 of endoderm cells on its lateral 

 surfaces. In subsequent de- 

 velopment these strips grow in breadth, and eventually, about the 

 time that the embryo hatches into a larva, the yolk is entirely sur- 

 rounded by a layer of endoderm cells and the mid-gut is complete. 



The sub-stomodaeal endoderuial mass, which is situated under the 

 stomodaeum, undergoes a strange fate : it becomes divided into right 

 and left halves connected by a narrow bridge of endoderm, and each 

 half becomes divided into anterior and posterior portions which 

 assume the form of rounded masses adherent to the volk. Cavities 



germ 



6 



FIG. 201. Diagram of sagittal section (rather to 

 one side of median line) through the embryo 

 of Dunacia crassiprs to show the coelomir, 

 saus. (After Hirschler.) 



Lc'tturs as in previous figure. In addition, un, rudi- 

 ment of antenna; coe.ali, coelomic sacs of the abdominal 

 segments ; coe.int, coelomic sac of the intercalary seg- 

 ment ; coe.th, coelomic sacs of the thoracic segments ; 1- 

 11, the abdominal segments ; gmn, primitive germ eel Is. 



