458 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 



originate around the stem just below the hydranths, and they are 

 themselves carried on short stems. The perisarc is not annulated, 

 and it forms a loose cylindrical sheath around the main stem and the 

 short branches which carry the lateral hydranths and the young 

 medusae, while the latter are invested by a much thinner and more 

 transparent capsule of perisarc. The sheath on the stem is thick and 

 crusted with foreign matter. It terminates abruptly by a sharp collar 

 just below each hydra nth. The young hydranths and the medusa.- 

 are budded off above the collar, but they soon become entirely sheathed 

 in perisarc by the growth of the stem. The pale yellowish-red hy- 

 dranths are very similar to those of Tubularia (Allman) and the hydroid 

 is so similar to Dendroclavd dohrnii recently described by Weismann, 

 that they undoubtedly belong to the same genus." 



SUMMARY. 



1. The ova of Turritopsis arise in the ectoderm of the manubrium. 

 They grow by the absorption of the primitive ovarian cells and when 

 mature are densely crowded with large yolk granules. 



2. Dehiscence takes place at a definite time, from 5 to 6 o'clock 

 in the morning. 



3. The egg is spherical and membraneless. It is composed of an 

 outer layer of clearer ectoplasm and a central mass of endoplasm which 

 is dense and opaque and filled with large, dark yolk spheres. 



4. Maturation and fertilization take place in the water after the 

 eggs are deposited. It is impossible to make out details in the living 

 eggs because of their opacity. 



5. Cleavage is total and nearly equal. The first three divisions 

 are fairly regular; but during the later segmentation the arrangement 

 of the blastomeres becomes very irregular and erratic. At the com- 

 pletion of segmentation a solid morula stage is formed, in which the 

 cell boundaries are lost for a time, giving rise to a syncytiurn. 



6. Parts of eggs which are divided during the cleavage stages con- 

 tinue to develop and form larvae which are normal in every respect 

 except size. 



7. The ectoderm is formed by the reappearance of cell walls in 

 the periphery of the syncytium mass, and is separated from the anterior 

 part by the formation of the mesogloea. 



