REPRODUCTIVE SYSTEM— DEVELOPMENT. 417 



pharynx, and are short tubes which place the sub-chordal crelome in 

 communication with the atrial chamber." They open into the ccelome 

 by three or four funnel shaped openings, and around them the vessels of 

 the gill slits form a so-called glomerulus. They occur in relation to the 

 gill slits, and open on the secondary branchial bars. Boveri regards 

 them as equivalent to the pronephric tubules of other Vertebrates. Of 

 their developmental history nothing is known. 



[b) Professor Ilatschek discovered in the anterior region of the larva 

 a nephridial tube which is absent in the full grown adult ; according to 

 Van Wijhe, this is the residue of the communication between the left 

 anterior diverticulum (or pre-oral pit) and the gut. 



(f) Professor Ray Lankester discovered a pair of short pigmented 

 funnel tubes (Fig. 133, a.c.f.), which lie in the twenty-seventh segment, 

 and place the lymphatic spaces of the metapleural folds in communica- 

 tion with the atrial cavity. They may be compared with the pores 

 which open from the collar region in Balanoglossus, and with the 

 abdominal pores of higher Vertebrates. "It is doubtful whether they 

 represent nephridia." 



Reproductive System. 



The sexes are separate and similar to one another. The 

 organs are very simple, and are without ducts. They form 

 twenty-six pairs of horse-shoe-shaped sacs, lying along the 

 inner wall of the atrial cavity in segments ten to thirty-five 

 on each side (Fig. 133, G.). Each lies in a "genital chamber" 

 formed in development by constriction from the cavity of 

 the lower part of the primitive segment. 



In the mature female the ovaries are large and con- 

 spicuous ; the ova burst into the atrial cavity, whence they 

 pass into the pharynx by the gill slits, and out by the mouth, 

 or more directly by the atriopore. 



The testes are like the ovaries ; the spermatozoa burst 

 into the atrial cavity, and pass out by the atriopore. The 

 eggs are fertilised in the surrounding water. 



Development. 



The fertilised ovum is about rta inch in diameter. The 

 segmentation is complete and almost equal. The first 

 cleavage is vertical, and divides the ovum into two equal 

 parts ; the second is also vertical, along a meridional plane 

 at right angles to the first, and the result is four equal cells. 

 The third cleavage is equatorial, and gives rise to four larger 

 cells (or macromeres) below or towards the vegetative pole, 

 and to four smaller cells (or micromeres) above or towards 



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