ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
515 
weapon or organ of attachment. In a second regeneration the tertiary 
tail agrees completely with the secondary tail. Within the same family 
the regenerated tails generally agree as regards the scaling. 
Hypotypic Regeneration.* — Prof. A. Giard finds in the phenomena 
of regeneration a tendency to reproduce ancestral conditions. The pen- 
tamerous tarsi of Blattid® and Phasmidae may be replaced by tetramerous 
tarsi, which are believed to be more primitive. The same tendency is 
seen in the scaling of lizards’ tails (Boulenger, Werner), in the dorsal 
appendages of Tethys lejporina (Parona), in the polydactylism of Axolotl 
and Pleurodeles , in the ringing of the stalk in Obelia (Davenport). To 
this kind of regeneration the author applies the term “ hypotypic,” i.e. 
returning to a lower type. 
Critical Period in Development of Horse.f — Prof. J. Cossar Ewart 
begins an interesting paper with a brief description of the foetal appen- 
dages in the horse, which he contrasts with those of bird and opossum. 
During the first seven weeks the embryo is fixed by the cells of the 
embryonic sac or trophoblast, and a special absorbing area is formed by 
a fusion between part of the yolk-sac and the outer sac. This acts as a 
sort of filter, through which the nutriment enters, and is absorbed by 
the vitelline blood-vessels. “ At the end of the third week of gestation, 
when the reproductive system passes through one of its periods of 
general excitement, about one-fourth of the embryonic sac probably 
adheres to the uterus ; but at the end of the sixth week, when another 
wave of disturbance arrives, all the grappling structures are at one pole. 
Hence there is probably more chance of the embryo ‘ slipping’ at the 
end of the sixth than at the end of the third week. About the end of 
the seventh week the supply of nourishment by means of the yolk-sac is 
coming to an end, and there is perhaps still about this time an hereditary 
tendency for the embryo to escape.” This is the most critical period ; 
its difficulties are overcome by the rapid sprouting of allantoic villi, 
which procure fresh supplies of food and oxygen, and effect firmer fixing 
of the embryo to the wall of the uterus. The paper concludes with 
some practical hints as to the precautions which may be taken to coun- 
teract the risks of the critical period. 
Origin of the Cleavage Centrosomes.J — Miss K. Foot discusses this 
difficult question with immediate reference to the egg of Allolobopliora 
fcetida. During the past two years the weight of authority has greatly 
increased in favour of the conclusion that the cleavage centrosomes are 
of spermatic origin. They are said to be the daughter centrosomes of 
the so-called male centrosome, which is believed to originate from the 
middle piece of the spermatozoon. But the phenomena of fertilisation 
in the egg of Allolobophora fcetida do not sustain this view ; they sug- 
gest that this centrosome has the same origin as the so-called egg-centro- 
some, and that both are cytoplasmic elements, of like origin and consti- 
tution. The sperm attraction-sphere and the fertilisation-cone have 
several points in common ; both structures appear to be dependent, not 
only upon the entrance of the spermatozoon, but also upon a definite 
* C.R. Soc. Biol., 1897, 3 pp. ; Zool. Centralbl., iv. (1897) p. G80. 
t London, Svo, 27 pp. and 2 pis. 
J Journ. Morphol., xii. (1897) pp. 809-14 (1 pi.). 
