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becomes perhaps a ciliated morula, this again a gastrula, with ciliated 
ectoderm and amoeboid endoderm ; this may settle down as in sponges, 
its cells re-cycling anew, the ectodermic layer becoming amoeboid, 
the endodermic ciliated (fig. 18). The endodermic cells remain per- 
manently more or less amoeboid, as the recently much investigated 
phenomena of intercellular digestion have so clearly established. 
The amoeboid ectodermic cells, on the other hand, may give rise to 
muscle — and a muscle is but an amoeba elongated so as permanently 
to contract along one line ; on the other hand, they may pass into 
a quiescent state, or throw out encysting material, which may either 
enclose them individually, as in Ascidians, or form a collective 
external envelope, as in Arthropods. The niesodermic cells may 
either remain unspecialised as amoeboid corpuscles, may specialise as 
muscular tissue, or cycle into the resting state, z’.e., develop into 
connective tissue (see fig. 19). 
And if the cell cycle persist thus long in the life-history of the 
organisms, why should it' disappear? In reality, it does not 
disa]3pear completely. The amoeboid corpuscles of the perivisceral 
fluid of an invertebrate — say an Echinus — develop, largely at least, 
from the ciliated epithelium lining of the coelome — permanently 
exhibit, that is to say, one of the most characteristic phenomena 
of the cycle. And when under proper precautions we examine 
a fresh drop of the fluid, we observe the corpuscles as they die 
running together into a plasmodium,* so perfectly similar to that 
of a Myxomycete as actually to have been described by a recent 
observer as a new genus and species, f And this phase of the cycle 
takes place, in the so-called coagulation of corpusculate fluids of inver- 
tebrates generally. Numerous other instances of the occurrence of 
some phase of the cell-cycle have been recorded, and have already 
been collected by the writer in a series of papers which have led to 
the present one ; it is unnecessary to call attention to others, save 
perhaps the especially interesting announcement by Professor Had- 
don,§ of the occurrence of a plasmodial union of cells during the 
normal histolysis of Polyzoa. 
* Geddes, “Observations s. 1. fiuide perivisceral des Oursins,” Arch. Zool. 
Exp. VIII. t OompUs Rendus, t. Ixxxii. ISTo. 21, 
J Geddes, “ On the Coag. of Amoeb. Cells into Plasmodia,” kc,., Proc. Roy. 
Soc. Loud., No. 202, 1880, and Trans. Roy. Rhys. Soc. Edin., 1882. 
§ Haddon, “ On Budding in Polyzoa,” Quart. Jour. Micros. ScL, 1883. 
