LOCALIZED STAGES IN DEVELOPMENT. 127 



to repeat more or less the characters of the- young from the egg, and therefore yield cases 

 of localized stages in development. 



Hydka. In Hydra, when tlie l)U(l is first formed, it is a simple outgrowth of tlie body 

 walls. As a new mouth breaks through and tentacles are formed, the growing H^vdra 

 passes through stages of development which may be compared with the later stages seen 

 in a Ilvdra i)roduced from the egg. In tlie budding of other Hydrozoa and Actinozoa, the 

 buds, when first foxnned, are simple, and during growth acquire their full specific (or 

 specialized in the case of specialized zooids) characters. Clark ('Go) figures the buds of 

 H^ydra. and describes the buds of a sea-anemone. In the latter, lie saj's the buds are at 

 first simple outgrowths, as in Hydra ; later a mouth and tentacles appear. At an early 

 stage there are onl}- six of these tentacles. 



Galaxea caespitosa. In ;i large specimen of Galaxea caespitosa Dana, from Singa- 

 pore, in the collection of the Boston society of natural history, there are many young 

 corallites budded out from the coenenchyma. In the youngest of these ))nils (PI. 24. fig. 

 106), there are only six septa. One of the septa is shorter than the others, probably due 

 to a break. In an older bud (PI. 21, fig. 107) adtUtional septa have appeared between 

 the six primaries. A still older specimen (PI. 24, fig. 108) shows a further development 

 of the six primary septa ; the secondary septa have also grown. In an adult corallite 

 from the same specimen (PI. 24, fig. 10'.)), the first twelve septa, including tlie primaries 

 and secondaries, have met and fused in the center. Additional tertiary septa exist 

 between the older septa, and are individually comparable to the early condition of the 

 primary and secondary septa, as seen in Figs. lOO-lOS^. 



The budding method of accpiiring characters which repeat stages seen in later devel- 

 opment from the egg, is distinctly a form of localized stages in development. It is (piite 

 different, however, from the cases given of localized stages of development in other ani- 

 mals, where the repetition of characters is seen in parts serially added during the growth 

 of one individual. 



Crinoidea. In crinoids, the stem grows ))y increase in size of the plates, more or less 

 throu";hout its lenii'th, and also hv the intercalation of new plates between the cahx and 

 to[)niost plate of the stem, or in that immediate vicinity. It is, therefore, seen that plates 

 near liie calyx are the youngest plates throughout the life; of the individual. 



\\\ the recent Metacrinua rotimdus Carp, from Japan, the plates close under the calyx 

 are strongly pentagonal; they are also thickly clothed with cirrlii (PI. 25, fig. 124). In 

 the older portion of the stem (Fig. 124a), the plates have increased in size, are nearly 

 circular in section, and the cirrhi occur as quite widely separated whorls. 



In the recent Pcutdcrinus dcconis Wy. Tlioms. a similar condition exists. The plates 

 close under the calyx are pentagonal and farther down are nearly round. In this species 



