IV 



VERMES ONTOGENY OF THE WORMS 



279 



limited germ zone represents the condensed rudiment of all mesodermal organs, with 

 the exception of the nervous system. 



We accordingly find in the Hirudinea (Cl-epsine) and in Lumbricus, in early 

 stages of development, even during 

 segmentation, on each side of what 

 morphologically corresponds with 

 the posterior end of the embryo, 4 

 or 5 micromeres, which soon sink 

 down under the ectoderm - micro - 

 meres, or are grown round by them. 

 The 2 which lie posteriorly are the 

 largest ; they lie close to each other, 

 and in front of them 3 or 4 lie on 

 each side of the ventral median line 

 (Fig. 188). The 2 inner micromeres 

 we already know ; they are the 

 neuroblasts. Just as 2 rows of cells 

 develop anteriorly from the neuro- 

 blasts, forming the rudiment of the 

 ventral chord, rows of cells also 

 develop anteriorly from the remain- 

 ing polar cells. The single or double 

 cell rows which lie near the neural 

 rows are the nephridial rows ; the 

 polar cells from which they develop 

 are the nephroblasts. They yield 

 the material for the nephridia, which 

 become differentiated from before 

 backward. The most anterior ne- 

 phridia are temporary ; they are the 

 larval or embryonic nephridia. It 

 is not yet known what part of the 

 mesoderm is formed by the lateral 

 rows of cells with their posterior 

 polar cells. The cell rows which 

 proceed from the most posterior lar- 

 gest polar cells represent the rudi- 

 ments of the body musculature, the 

 endothelium, septa, and mesenteries. 

 We shall return to their further 

 development. 



All these cell rows taken together 

 are known as the paired germ streaks. 

 They lie at the two sides of the ven- 

 tral middle line between the intestine 

 and the integument. The germ 



pme 



FIG. 189. Surface view of the germ streaks of a 

 somewhat older Lumbricus-embryo, after the disap- 

 pearance of the anterior polar cells (after Wilson). 

 pme, Polar cells of the mesoderm (m) ; x, xr, cell rows 

 of unknown significance ; nphr, nephridial cell streaks ; 

 streaks thus yield all the mesodermal nbr, neural cell streaks, rudiment of the ventral chord 

 organs, nervous system, nephridia, ( bm ) ', d ^ dissepiments ; ib, inner ; ab, outer rows of 

 muscles, endothelium, etc. In the seti P arous S^ds; nph, rudiments of the nephridia; 

 .. T 7 7 *, . 7 i .1 11 mu > longitudinal musculature. 



Ghiathobdefiidce, where the whole 



larval ectoderm is lost, the germ streaks are said even to produce the definitive 

 body epithelium as well. This statement, however, requires confirmation. 



