Miscellaneous. 65 
the nuclei and the formation of the granulosa. I can only compare 
this series of phenomena to the migrations observed by Pfltiger and 
Lindgren in the cells of the granulosa of the higher Vertebrata. 
The presence of an abundant nutritive vitellus (of an orange 
colour) gives rise in our Lithonephria to a remarkable condensation 
of the embryogeny. I shall indicate only two particularly inter- 
esting stages. 
At the stage VIII. the ovum presents four coloured endodermic 
cells and four colourless exodermic cells, arranged as in typical cases 
of epibolism. 
At the stage XX XII., and even earlier, the ovum clearly displays 
the bilateral symmetry of the adult; at the nutritive pole we see 
six endodermic blastomeres, two large and four smaller ones. At the 
base of the two large ones six mesodermic blastomeres form a half- 
equator: three mesodermic spheres are situated to the right of the 
plane of symmetry, and three to the left; the spheres increase in 
size from this plane. 
At the formative pole twenty cells constitute an exodermic hemi- 
sphere: twelve are arranged in two series of six on either hand of 
the meridian of symmetry ; the others form two groups of four cells 
each, occupying the free space to the right and left between the 
endoderm and the exoderm. 
The study of the segmentation shows that the six mesodermic 
blastomeres are derived from two spheres which themselves issue 
from the endoderm, and are situated symmetrically with relation to 
the median plane at the point of junction of the endoderm and ex- 
oderm. 
The six mesodermic cells are afterwards covered by the exo- 
dermic cells in consequence of the progress of the epibolism ; they 
also become more numerous ; the half circlet contracts and acquires 
the form of a horseshoe. This is the rudiment of the chorda, so 
characteristic thatit hasstruck all the authors who have paid attention 
to the embryogeny of the Ascidians; but in the ova with equal 
segmentation this rudiment appears much later. 
I have often insisted upon the point that, in unequal segmenta- 
tion from the stage IV., the ovum at the stage VIII., which is physio- 
logically a morula, morphologically represents a gastrula. In the 
case now before us the ovum at the stage XXXII. is still physiolo- 
gcally a morula; morphologically it already posseses a middle layer 
-(solid mesoderm), and represents a much more advanced stage of 
the Ascidians with dilated embryogeny. The embryogenetic con- 
densation might therefore be defined an advance of the morphological 
upon the physiological state of the embryo. . 
Here, as in all known cases, the solid mesoderm prodticed by two 
cells derived from the endoderm at the periphery of the prostoma 
(circle of contact of the exoderm and endoderm) appears before the 
cavitary mesoderm (enteroccele, cceloma, &c.). The former gives 
origin to the skeletal and muscular organs; the other forms princi- 
pally the hematic apparatus and the serous membranes properly so 
called. 
Ann. & Mag. N. Hist. Ser. 5. Vol. viii. 5 
