Sedentaria. The development of the fixed composite Ascidians is, so far as we know, in the main similar to that of the simple Ascidians. The larvæ of Botryllus sometimes attain, while still in the free state, a higher stage of development with reference to the number of gill slits, etc. than that reached by the simple Ascidians, and in some instances (Botryllus auratus Metschnikoff) eight conical processes are found springing in a ring-like fashion around the trunk. The presence of these processes has led to somewhat remarkable views about the morphology of the group; in that they were regarded by Kölliker, Sars, etc. as separate individuals, and it was supposed that the product of each ovum was not a single individual, but a whole system of individuals with a common cloaca.
The researches of Metschnikoff (No. [32]), Krohn (No. [25]), and Giard (No. [12]), etc. demonstrate that this paradoxical view is untenable, and that each ovum only gives rise to a single embryo, while the stellate systems are subsequently formed by budding.
Natantia. Our knowledge of the development of Pyrosoma is mainly due to Huxley (No. [16]) and Kowalevsky (No. [22]). In each individual of a colony of Pyrosoma only a single egg comes to maturity at one time. This egg is contained in a capsule formed of a structureless wall lined by a flattened epithelioid layer. From this capsule a duct passes to the atrial cavity, which, though called the oviduct, functions as an afferent duct for the spermatozoa.
The segmentation is meroblastic, and the germinal disc adjoins the opening of the oviduct. The segmentation is very similar to that which occurs in Teleostei, and at its close the germinal disc has the form of a cap of cells, without a trace of stratification or of a segmentation cavity, resting upon the surface of the yolk, which forms the main mass of the ovum.
After segmentation the blastoderm, as we may call the layer of cells derived from the germinal disc, rapidly spreads over the surface of the yolk, and becomes divided into two layers, the epiblast and the hypoblast. At the same time it exhibits a distinction into a central clearer and a peripheral more opaque region. At one end of the blastoderm, which for convenience sake may be spoken of as the posterior end, a disc of epiblast appears, which is the first rudiment of the nervous system, and on each side of the middle of the blastoderm there arises an epiblastic involution. The epiblastic involutions give rise to the atrial cavity.
These involutions rapidly grow in length, and soon form longish tubes, opening at the surface by pores situated not far from the posterior end of the blastoderm.
Fig. 12.
A. Surface view of the ovum of Pyrosoma not far advanced in development. The embryonic structures are developed from a disc-like blastoderm.
B. Transverse section through the middle part of the same blastoderm.
at. atrial cavity; hy. hypoblast; n. nervous disc in the region of the future Cyathozooid.
The blastoderm at this stage, as seen on the surface of the yolk, is shewn in [fig. 12] A. It is somewhat broader than long. The nervous system is shewn at n, and at points to an atrial tube. A transverse section, through about the middle of this blastoderm, is represented in [fig. 12] B. The epiblast is seen above. On each side is the section of an atrial tube (at). Below is the hypoblast which is separated from the yolk especially in the middle line; at each side it is beginning to grow in below, on the surface of the yolk. The space below the hypoblast is the alimentary cavity, the ventral wall of which is formed by the cells growing in at the sides. Between the epiblast and hypoblast are placed scattered mesoblast cells, the origin of which has not been clearly made out.