CHAPTER II.
UROCHORDA[5].
In the Solitaria, except Cynthia, the eggs are generally laid, and impregnation is effected sometimes before and sometimes after the eggs have left the atrial cavity. In Cynthia and most Caducichordata development takes place within the body of the parent, and in the Salpidæ a vascular connection is established between the parent and the single fœtus, forming a structure physiologically comparable with the Mammalian placenta.
Solitaria. The development of the Solitary Ascidians has been more fully studied than that of the other groups, and appears moreover to be the least modified. It has been to a great extent elucidated by the splendid researches of Kowalevsky (Nos. [18] and [20]), whose statements have been in the main followed in the account below. Their truth seems to me to be established, in spite of the scepticism they have met with in some quarters, by the closeness of their correspondence with the developmental phenomena in Amphioxus.
The type most fully investigated by Kowalevsky is Ascidia (Phallusia) mammillata; and the following description must be taken as more especially applying to this type.
The segmentation is complete and regular. A small segmentation cavity appears fairly early, and is surrounded, according to Kowalevsky, by a single layer of cells, though on this point Kupffer (No. [27]) and Giard (No. [11]) are at variance with him.
Fig. 5. Transverse section through the front end of an embryo of Phallusia mammillata. (After Kowalevsky.)
The embryo is slightly younger than that represented in fig. 8 III.
mg. medullary groove; al. alimentary tract.