VIII 



AETHEOPODA 



181 



of the blastodermic cells from a flattened to a more columnar shape. 

 The cephalic lobes, which have increased in size, are brought nearer 

 to each other so that they are only separated by a groove, and they are 

 also approximated to the thoracico-abdominal rudiments. The endo- 

 dermic disc is indented in its anterior portion by a deep, semicircular 

 groove ; this groove is the beginning of the process of gastrulation 

 (Figs. 129 and 130), and may be termed the endodermic groove. 



The mesoderm which lies in front of this consists of a limited 

 number of large cells termed primary mesoderm, mingled with a 

 larger number of small cells. The former will give rise to masses 

 corresponding to the somites of Feripatus, from which the muscles 

 and probably the genital organs arise ; the latter constitute Eeichen- 

 bach's so-called secondary 

 mesoderm, they wander 

 widely and occur every- 

 where between ectoderm 

 and endoderm, and appear 

 to give rise to blood and 

 connective tissue cells. 

 Eeichenbach emphasizes 

 the fact that these cells 

 originate both from ecto- 

 derm and from endoderm, 

 but it seems probable that 

 the primary mesoderm has 

 an endodermic origin, while 

 the secondary springs from 

 the ectoderm. 



Soon the endodermic 

 groove becomes a complete 

 circle and the periphery 

 of the endodermic disc is 

 invaginated. Just as we Pk-. 129 

 have found to be the case 

 in other eggs, the process 

 of invagination can be 

 analysed into (a) an in- . ^ ^ • 



crease in the number of cells and (b) an inwardly directed cytotaxis. 

 The result of this kind of process is that the centre of the endo- 

 dermic disc projects for a time as a kind of endodermic button, hut 

 as the process continues this button is also carried inwards, and a 

 circular blastopore is left where once there was a superficial disc 

 of endoderm. The anterior part of the periphery undergoes the 

 most rapid invagination, and so the endodermic sac projects forward 

 beneath the thoracico-abdominal rudiments. 



These rudiments are now connected with one another by a bridge 

 of hich columnar cells, and each is also connected with the cephahc 

 lobe of its side by a streak consisting of parallel lines of columnar 



th.ahd 



inv 



-Ventral view of an embryo of Astacus 

 flumalilis, the gastrula stage, in order to show the 

 ventral plate. (After Eeichenbaeh. ) 



c.l, cephalic lobe ; inv, invaginated area of blastoderm ; 

 th.dbd, thoracico-abdominal thiolcening. 



