TROCHOPHORH OF HYDROIDES T'NCIXATUS (eUPOMATUS). 571 



I will now describe in detail tlie cbaiiges imdergone by 

 tbese cells. In a late gastriila stage such as tbat sbowu in 

 this text-figure (Text-fig. 18) tbese cells have already divided ; 

 the division usually is an unequal oue, in wbicb one of the 

 daughter-cells is mucb suialler and moreirregular in shape than 

 the other. Tliey seldom divide simultaneously on both sides, 

 buc tbe right usually precedes the left. If we I'efer to fig. 9, 

 we see the larger of these cells attached to the ventral wall 

 of the oesopbagus [hk.). The smaller seems to give rise to 

 some of tbe mesenchyme cells tbat are attached to the wall 

 of the oesophagus. Tbese are very irregular in shape and 

 size. At this early stage they are only seen with difficulty, 

 as they are few in number, and are closely pressed against tbe 

 surface of the oesophagus. Altbougb tbe stage represented 

 in fig. 9 has already assumed the shape of tbe early trocbo- 

 phore, it is but slightly older tban tbe late gastrula stage 

 represented in Text-fig. 18. 



The bead-kidney strand is derived from the division of tbe 

 large cell (liJi.) seen in fig. 9. This divides once, and tben by a 

 second division of oue of tbe daughter-cells a band of three 

 cells is formed (Text-fig. 23). The nuclei of tbese cells so 

 arrange themselves that two remain in tbe end of tbe strand 

 attached to tbe oesophagus, while one moves to tbe distal 

 end, Avbich abuts against tbe anal end of tbe gut. This stage 

 is represented in fig. 12. In fig. 10 tbe head-kidney cell has 

 divided, forming two daugbter-cells, one of Avhicli is applied 

 close to the Avail of the oesophagus, while the otber rests 

 agaiust tbe inner lower surface of the larval bemispbere. 

 In fig. 12 the two distal nuclei of tbis band have moved 

 apart, one resting against tbe anal end of the gut, while tbe 

 otber remains close to the oesophagus. 



In Text-figs. 21 and 22 tbe strand of cells forming tbe 

 head-kidney is shown in sections. In Text-fig. 21 {coe. m.) is 

 show-n part of the nucleus of the coelomesoblast cell. This 

 figure is almost in tbe median plane, while the plane of section 

 of Text-fig. 22 is quite oblique, showing only a portion of the 

 stomach and gut-wall. The cell boundaries disappear, so 



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