243 
Development of Pyrosoma. 
I was not able to discover any connexion between the axial 
canal of the mesoderm and the enteric cavity, nevertheless 
the position of this canal renders it extremely probable that 
we have in it the nearest representative of the chorda dorsalis 
of the Ascidian embryos. The beautiful investigations of 
E. van Beneden and Julin have taught us that the notochord 
of the embryos of Clavellina is, in the earlier stages of deve- 
lopment, represented by a tube which is situated between the 
two mesodermic diverticula, and therefore exhibits the same 
relations as we find in the axial tube of the embryos of 
Pyrosoma. In the case of Pyrosoma the tube in question is 
a transitory structure and lasts but a very short time. 
In spite of the similarity between the coelomic sacs of 
Pyrosoma and those of the Ascidian embryos, to which I have 
just alluded, the two structures nevertheless exhibit an 
important difference in their histology. This consists in the 
fact that, while the coelomic sacs of the Pyrosoma- embryos 
are bounded by a multilaminar tissue, those of the Ascidian 
embryos have unilaminar epithelium-like walls. In the later 
stages of the Pyrosoma-embryos, however, this difference is 
removed ; for, in the course of growth, the coelomic sacs 
likewise become bounded by a single layer of cells. The two 
sacs, right and left, are at first precisely similar, and are 
symmetrically placed with regard to the longitudinal axis of 
the germinal disk. This condition, however, is of but short 
duration. As early as the stage of the first appearance of the 
peribranchial tubes, the two sacs exhibit important differences 
trom one another ; with this a second period in the develop- 
ment of the coelomic sacs is inaugurated, which may be termed 
the metamorphosis of the coelomic sacs . While the left coelomic 
sac has greatly increased in size and forms a spacious cavity, 
the cavity of the right sac has almost entirely disappeared 
and appears as a tiny lacuna which adjoins the septum 
between the two primitive coelomic spaces. The entire distal 
portion of the right coelomic sac is now represented by a 
solid mass of cells, from the periphery of which some cells 
are in the act of being liberated. In the stage at which the 
peribranchial canals deepen and form little blind tubes, the 
whole of the right coelomic sac has completely disappeared 
and has broken up into little cells. The development of the 
left coelomic sac, on the contrary, rapidly proceeds. The 
circumference of the sac increases, and it subsequently divides 
into two portions : its proximal portion becomes incrassated, 
and forms a swelling situated beneath the endostyle which is 
in process of formation ; in its further development it plays 
a very important part in the formation of various mesodermal 
