THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL 



Vol. III. 



New York, October, 1882. 



Xo. 10. 



Developnient of the Planula of 

 Clava Leptostyla, Ag.* 



BY J. H. PILLSBURY. 



On the tangled masses of sea-weed 

 {Fucus) which grow in profusion over 

 the rocks of our New England coast 

 between high and low water, may be 

 found little clusters of the orange- 

 yellow polypites of one of our com- 

 monest hydroids, Clava leptostyla, Ag. 

 Fig. I of the plate represents a portion 

 of the sea-weed with such a cluster of 

 polypites, of natural size. As the in- 

 coming or receding tide floats the 

 long stems of the sea-weed back and 

 forth over the rocks, the little animals 

 may be seen gracefully moving with 

 the waves, or suddenly contracting 

 their tentacles on the approach of 

 danger. 



All the polypites of the same clus- 

 ter are connected at the base by 

 minute, thread-like fibres, called sto- 

 lons, which unite them into a single 

 colony. Each polypite consists of a 

 tube with an oral opening at the free 

 end, which is surrounded for some 

 distance down the slightly swelling 

 body with a number of stout, and 

 somewhat scattered, tentacles (fig. 2). 

 Just below these tentacles the body 

 contracts into a perceptibly narrower 

 stem, which maintains a uniform size 

 until just above its union with the 

 stolon. At the point of contraction 

 just mentioned there will be found in 

 early summer, of various sbapes and 

 sizes, clusters of minute buds, which 

 often extend down the stem for a dis- 

 tance of 2 ""». These buds (fig. 2 a) 



* Read before the Section of Histology and 

 Microscopy of the A. A. A. S., at Montreal. 



are formed by the outward growth of 

 the body-wall, and at first do not dif- 

 fer in appearance fifom the general 

 tissue of the body. 



Usually before the bud has reached 

 its full size, one or two small, trans- 

 parent spots appear in the outward 

 and larger end of the bud (fig. 3 a 

 and l>). These are the germinal vesi- 

 cles, and each contains a distinct 

 spot called the germinal dot. This I 

 have been able to distinguish only by 

 the aid of carmine staining, but under 

 such treatment it becomes very dis- 

 tinct. As the buds continue to grow 

 the sarcode surrounding this germinal 

 vesicle becomes more dense than that 

 in other parts of the bud, and a little 

 later the ovum separates from the 

 general mass of the bud, which is 

 now called the spadix (fig. 4 a and 

 d). The ovum continues to increase 

 in size at the expense of the spadix 

 until it nearly fills the sac, the spadix 

 being meanwhile reduced to a cup- 

 shaped termination of the stem of the 

 sporosac. Such of the buds as had 

 two germinal vesicles will, of course, 

 under favorable circumstances, de- 

 velop two ova within the same 

 sporosac (figs. 4 ^ and 5 d). 



When the ovum has reached its 

 full size and fills the whole sporosac, 

 the ovum becomes so opaque as to 

 make the germinal vesicle obscure, 

 and render observation in regard to 

 its changes impossible. The ovum 

 itself, however, soon divides into two 

 cells, the division commencing by a 

 depression at one side, which becomes 

 gradually deeper and deeper until the 

 two cells are entirely separated (figs. 7 

 and 8). Each of these cells again di- 

 vides into two, and this is repeated 



