THE DEVELOPMENT OP ASTEEINA GIBBOSA. 253 



The History of the Young Star-fish. 



The just metamorphosed Asterina gibbosa has a disc of 

 about -6 millimetre in diameter ; if we take R to denote the 

 length from the tip of the arm to the centre of the disc, then 

 R equals SQ millimetre. A larva such as that figured in 

 figs. 51—53 may be -8 millimetre from the tip of the adhesive 

 disc to the posterior end, and measured obliquely from the 

 dorsal end of the prseoral lobe may exceed a millimetre in 

 length. There is, therefore, a considerable diminution in size 

 during the metamorphosis, the reason of which is evident when 

 we consider that no nutriment is taken during this time. A 

 full-grown specimen may have a diameter one hundred times 

 greater than that of the just metamorphosed star-fish, — that is, 

 it may exceed the latter one million times in bulk. The young 

 star-fish, however, rapidly increases in size, and by the time 

 R equals 3'7 millimetres the ovaries are visible. This is the 

 oldest stage I have examined; my account of the histology is, 

 however, taken from smaller specimens, in whichRequals 'Smm. 



The changes we shall have to consider are (1) the formation 

 of the primitive germ cells, the ovoid gland, genital rachis, and 

 ovaries; (2) the dermal branchise; and (3) general histological 

 differentiation. 



We have already in Fig. 76 seen the first trace of the ovoid 

 gland. It there appears as a ridge projecting into the axial 

 sinus ; inside this ridge there is as yet to be found only amoebo- 

 cytes, jelly and fibres, as is the case with the other blastocoelic 

 spaces in the larva. Later, a thickening of peritoneum takes 

 place on the wall of the left posterior coelom opposite the aboral 

 end of this ridge — and from this thickened patch a cord of cells 

 grows into the ridge, gradually forcing its way in an oral 

 direction ; this is the characteristic core of the ovoid gland. 



From this same thickening of peritoneum a cord of cells 

 grows out in a direction parallel to the disc ; this is the origin 

 of the genital racMs. By the outgrowth of a flap of peritoneum 

 it is enclosed in a space which is called the aboral sinus. The 

 genital rachis and the space enclosing it both give off branches 



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