320 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
At first there arc, on the right side, fourteen primary slits, none 
closing ; above these are six secondary thickenings ; the endostyle is in 
front of all the slits and the atrium is widely open anteriorly. On the left 
side there is a large lateral mouth, and one to two elements of the buccal 
skeleton. In the next three stages we find the fourteenth slit (on the 
right side) become closed, the thirteenth closing and the first very small ; 
the secondary thickenings become slits, and some commence to form 
tongue-bars ; the endostyle extends a short way backwards ; and the 
atrium becomes closed. On the left side, we find the mouth bending 
round to the middle line, and the oral hood and the cirri beginning to 
make their appearance. 
In the stage called the fifth there are twelve primary slits, just visible 
at the base of the pharynx, and the twelfth is closing ; the first undergoes 
atrophy ; there are eight secondary slits, the larger of which have 
complete tongue-bars ; the endostyle extends further back. On the 
left side the primary bars have not yet quite appeared, while the upper 
and lower portions of the oral hood have joined. The club-shaped gland 
of the right side undergoes atrophy. 
In the three succeeding stages the gill-slits tend more and more to 
arrange themselves in a bilaterally symmetrical manner, and there 
are at last eight on each side ; the endostyle increases gradually in 
size. 
The author gives a useful summary of the history of the 
individual structures, and then proceeds to certain general considera- 
tions. He thinks that the remarkable asymmetry of the larva may 
be ultimately traced to the adaptive forward extension of the notochord ; 
the asymmetry is, then, a purely ontogenetic phenomenon, and is not an 
ancestral character. Eeasons are given for regarding the club-shaped 
gland as a modified gill-slit of the right side, the corresponding slit of 
the left being represented by the first primary slit. 
With regard to the endostyle, evidence is adduced to show that its 
position in the adult Amphioxus is secondary, and that, in its origin, it is 
perfectly homologous with the endostyle of Ascidians. 
Some evidence is adduced in support of the startling assertion that 
the gill-slits or branchial stigmata of the Ascidians are not homologous 
with those of Amphioxus in origin, position, or relations. In the latter 
the slits are formed metamerically in the segmented region of the trunk ; 
in the former they appear in front of the segmented region, and do not 
arise metamerically but irregularly. The common ancestor of the 
two groups cannot be properly imagined till we have further knowledge 
as to the significance of the primary pair of diverticula of the prechordal 
vesicle, and as to the function of the club-shaped gland. It is probable 
that gill-slits were present in the segmented region of the trunk and 
have been lost by existing Ascidians ; if the evidence as to the club- 
shaped gland being a modified gill-slit is accepted, there must have been 
at least one pair of such glands. 
Development of Muscular Fibres.* — M. L. Roule has studied, 
chiefly in Porcellio, the development of striated muscular fibres. Some of 
the elements of the mesoderm which are arranged in compact groups on 
* Comptcs Itendus, cxii. (1891) pp. 245-6. 
