280 COMPARATIVE ANATOMY CHAP. 



There are germ streaks in many worms similar to those in the Hirudinea and 

 Liiiiilii'ii'ns. But it appears that these do not consist of contiguous rows of cells, lint 

 that each germ streak is the product of a single polar cell at the end of the embryo 

 or young larva. We have described these two polar cells in Eiqwmatus as primitive 

 mesoderm cells. The germ streaks which proceed from them are called mesoderm 

 streaks ; they seem to yield the whole mesoderm with the exception of the nervous 

 system. Here, therefore, the rudiments of almost all the mesodermal organs are 

 localised and condensed into two blastomeres which appear at an early stage at the 

 posterior edge of the blastopore. 



In the larva of Lopadorhynchus (PoUjclio:ta}, as the trunk grows, what is called the 

 postoral ventral plate of the ectoderm is said to split from front to back into an 

 inner muscular plate and an outer neural plate, the ventral chord being principally 

 formed from the latter. 



Further Development of the Germ Streaks (Figs. 187-189). In Lumbricus 

 the two cell rows of the germ streaks, which are formed from the two large pos- 

 terior polar cells, soon begin to be differentiated from before backward. Their 

 cells divide. The simple cell rows thus become solid plates or strands. They 

 extend forwards on both sides and meet dorsally above the mouth. In this 

 cephalic portion of the germ streaks a central cavity appears which enlarges and 

 becomes the cavity of the head. The outer layer of cells attaches itself to the outer 

 integument of the head, and becomes the outer musculature and endothelium of the 

 head segment. The inner layer forms the musculature of the pharynx and its 

 endothelial covering. Behind the head on each side clefts also occur in the above- 

 mentioned cell strands, and these, forming in segmental order from before backward 

 and increasing in size, form the rudiments of the paired segmental chambers 

 of the ccelome. They separate the cell strands in each segment into a parietal 

 layer contiguous to the integument and a visceral layer contiguous to the intestine. 

 The cell strands are thus divided into paired segmentally consecutive portions. 

 In each segment on each side both the parietal and the visceral layers grow up 

 between integument and intestine till they meet in the dorsal middle line. The 

 partition wall which is here formed in this way, and which divides the two lateral 

 chambers of the body cavity, is the rudiment of the dorsal mesentery which is often 

 temporary. A ventral mesentery arises in a similar manner. The cell material, 

 which separates the consecutive chambers of the body cavity forms the dissepiments. 

 The parietal layer forms the musculature of the body wall and the parietal 

 endothelium ; the visceral layer forms the muscular layer of the intestine and the 

 visceral endothelium. 



Development of the Blood-vascular System. This system arises (TcrcbcUa, 

 Psyymobranchus) as a cavity filled with fluid between the epithelial intestinal wall 

 and the contiguous visceral layer of the mesoderm. The chief vessels arise as longi- 

 tudinal bulgings of the mesodermal walls of this enteric sinus, which finally become 

 constricted off from it, passing from the form of grooves to that of closed canals. In 

 Liiiiibriciis and other Oligoclmta the dorsal vessels arise in a similar way as clefts 

 between the enteric epithelium and the enteric muscle layer. They are at first 

 paired, but generally unite to form the unpaired dorsal vessel. In a few Lumbricidce, 

 however, even in adult animals, over a larger or smaller region of the body, they 

 may remain double. 



Development of the Nephridia. In the anatomical section we distinguished 

 three parts in each nephridium : (1) the funnel ; (2) the nephridial duct ; and (3) the 

 terminal portion which opens outwardly, which in the Hirudinea and Oligochccta is 

 often widened out into a vesicle. It appears that in Lumbricus the nephridia 

 develop in the following way. The nephridial ducts develop in pairs in each 



