1885.] Embryology. 1233 
now state. 
Its minuteness might very readily lead to its being overlooked, 
It measures alive .5™™ in height; the longitudinal sections in my 
possession are somewhat shorter, or about .475™™ in length, a 
difference probably due to contraction. Its diameter at the oral 
end is somewhat greater than at the base, and ranges from about 
.15 to.175™™ In fractions of inches its dimensions are: height zy 
inch, diameter of body +45 inch, constituting it the most diminutive 
adult hydroid or ccelenterate type yet known, so that its generic 
name is especially appropriate. In life the animal is very much 
less contractile than Hydra, and is without the well-defined pedal 
disk of the latter or of Protohydra. 
No sexually mature individuals of Microhydra have been 
found by Mr. Potts, who has, however, most laboriously and care- 
fully studied the gemmiparous or asexual reproduction of the 
species in the living animal, he having been able, in fact, to watch 
this process in several successive generations, The asexual re- 
production of Microhydra is, however, so very different from that 
of Hydra that there can again be no doubt of the absolute dis- 
tinctness of the two forms in question. In Hydra the gemmipa- 
rously produced young animal, which is budded from the sides of 
the body, soon becomes pyriform, buds out tentacles and is con- 
stricted off from the parent by transverse fission around its base 
or pedal disk. In Microhydra a very different method of lateral 
gemmation occurs, since the bud very soon becomes sausage- 
shaped instead of pyriform, but it grows out from a swelling 
or thickening at the side of the parent the same as in Hydra. 
In Microhydra a constriction or furrow appears at either end of 
the young bud as soon as it is well defined as such. The gradual 
aaa ane 
_ progress of the development of a bud is shown in Figs. a, 4, c, d, 
e,f and g. The last shows the young Microhydra in the act | 
