1G2 



EMBRYOLOGY. 



would become fragmentary and the comprehension of the separate 

 processes would be made more difficult ; but it is necessary, in the 

 interest of a didactic method, to select from all the manifold pheno- 

 mena a single process of the development, and to follow it up until 

 it has come to a preliminary termination. 



After the formation of the middle germ-layer two important 

 processes take place in the embryonic fundament. One process 

 leads to a division of the middle germ-layers into the two lateral 



A %^2 



B 



en 



- ud 



Fig. 103. Amphioxus embryo with five pairs of primitive segments in optical section, after 



HATSCHEK. 



A Seen from the side. B Seen from the dorsum. 

 In figure B are indicated the openings of the cavities of the primitive segments into the 



intestinal cavity, which can be seen by deeper focussing. V, Anterior, &, posterior end ; 



a/.-, outer, ik, inner, mk, middle germ-layer ; dh, intestinal cavity ; n, neural tube ; 



en, neurenteric canal ; us 1 , first primitive segment ; ush, cavity of primitive segment ; 



ud, ccelenteron. 



plates and into two series of cuboidal bodies, which are situated at 

 the right and the left of the chorda, and which, under an erroneous 

 interpretation, were formerly called prolovertebrce, but for which one 

 should now substitute exclusively the more accurate name primitive 

 segments [mesoblastic somites]. The other process, which occurs at 

 about the same time, at least in the case of the higher Vertebrates, 

 leads to the origin of those cells from which the sustentative sub- 

 stances and the blood of Vertebrates are derived. 



