338 EMBRYOLOGY 



These wo rows of macromeres have been compared to the macromeres 

 of the Hirudinea, though, as far as can now be seen, this comparison is 

 ijot warranted, for the macromeres in Branchiobdella are said to continue 

 dividing, and to give rise to the mesoderm and entoderm. But in both 

 the Hirudinea and the OHgocheBta the separation of the two germ -layers 

 takes place much earlier. 



The division of the macromeres advances steadily from 

 behind forwards. In this way two different groups of cells 

 arise, one of which lies next to the ectoderm, and constitutes 

 the mesoderm, while the other, lying next to the macro- 

 meres, is the entoderm. What is left of the macromeres 

 themselves divides uninterruptedly, so that the cells arising 

 in this way become like the ectoderm. They cover the 

 posterior part of the embryo (Fig. 157 D). 



Even before the macromeres separated into the different 

 elements in the manner described, a depression of the ecto- 

 derm (Fig. 157 A), which does not persist long, and perhaps 

 represents the fundament of the supra-oesophageal ganglion, 

 makes its appearance in front of them, and therefore on the 

 dorsal side of the embryo. This originates independently of 

 the ventral chain of ganglia. The latter arises, according to 

 Salensky, in the form of an extensive groove on the ventral 

 side (Fig. 157 C). The groove is very broad at the posterior 

 end of the embryo. It is bounded here by the large cells 

 still remaining, which, continuing to divide, contribute to 

 the formation of the margins of the groove. The groove 

 becomes narrower anteriorly, extends on to the dorsal side 

 of the embryo, and here bifurcates (Fig. 157 D). The part 

 of the ectoderm which is encircled by the two branches 

 probably corresponds to the above-mentioned ectodermal 

 depression, and produces the supra-oesophageal ganglion, 

 which secondarily unites with the two processes of the ven- 

 tral chain of ganglia by means of two ridge-like processes, 

 the oesophageal connectives. The ventral cord itself is said 

 to originate in a manner quite like that of the medullary 

 tube of vertebrates. The groove becomes closed by the 

 bending together of its upper edges (in this case, however, 

 from in front backwards), and in this way forms a tube, 

 which finally separates from the overlying ectoderm, loses 



