ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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peripheral layer of the ovum has a firmer consistence than the rest of 
the egg ; an exaggeration of this forms the yolk-membrane : the larvae 
are enveloped by an analogous pellicle; around the egg it increases in 
firmness after formation ; the spermatozoon probably stimulates the 
ovum to a slight chemical change which results in the delimitation and 
hardening of the plasmic pellicle ; so do the reagents noted above. 
Probably, as Fol suggests, there is formed between the membrane and 
the ovum some gelatinous substance, which coagulates in the water and 
raises the membrane. Eggs whose membrane has been shaken off may be 
stimulated by chloroform, &c., to form another, or a double one may be 
evoked around fertilized ova. There is no doubt that the conditions 
producing the membrane are normally within the ovum itself, and it is 
likely that the spermatozoon supplies the requisite stimulus, analogous 
to that due to the reagents. 
Cuvierian Organs.* — Prof. H. Ludwig and Herr P. Barthels have 
studied these in ten species of Holothuria and in Miilleria mauritiana 
(Quoy and Gaim.). In the last named species and in Holothuria Kollikeri 
Semp., the organs are branched ; in the nine other species of Holothuria 
they are simple and caecum-like. The caecum-like organ has an axial 
canal, which Semper denied, and the following layers: an internal 
epithelium, an internal connective-tissue layer, circular muscles, longi- 
tudinal muscles, an external connective-tissue layer, and wandering cells 
in both layers of connective tissue, and a glandular epithelial layer 
apparently derivable from the original coelomic epithelium around the 
organs. The axial canal of the tubules communicates with the lumen of 
the respiratory tree ; and Herouard seems to be right in regarding the 
Cuvierian organs as modifications of branches of the respiratory tree, 
the structural resemblance being very close. Instead of dividing 
Cuvierian organs into branched and unbranched, the authors would 
make the contrast between glandular and non-glandular. The glandular 
organs are always unbranched, at least no certain exceptions are known: 
the non-glandular organs are either unbranched (Molpadia chilensis 
J. Miill., Mulleria maculata Br.), or branched ( Miilleria lecanora Jag., 
M. mauritiana Quoy and Gaim., Holothuria Kollikeri Semp.), and have 
stalked vesicles on their surface (except in Miilleria maculata'). Their 
function remains a riddle. 
Deposits of Synaptidse.f — Prof. H. Ludwig remarks that it may 
seem strange that there is anything new to say about these structures, 
which have been described so often. He begins with a description of 
those of Chiridota pisanii, traces their development from a minute 
hexagonal star, and compares them with those of other forms. Synaptidse 
with plates may be arranged in two groups — (a) those with solid “ nave ” 
without a lid ( Myriotroclms , Acanthotrochus , and perhaps Trochoderma ), 
and (b ) those with hollow “ nave ” and with a lid ( Chiridota and Trocho- 
dota). Thus there is a Myriotroclms group and a Chiridota group. The 
other Synaptidas, in which the plates are absent in adult life, may be 
spoken of as the Synapta group, but they are linked to Chiridota through 
Anapta. 
* Zeitschr. f. Wiss. Zool., liv. (1892) pp. 631-54 (1 ph). 
f Tom. cit., pp. 350-64 (1 pi.). 
