4 Tun. 
VI. TUNICATA. 
b. Embryology. 
[Including budding and changes in the colony.] 
Caullery describes the origin of the buds in Colella. He finds a 
polymorphism, some buds having an abundant store of reserve materials 
in the outer layer, while others are without this supply. The former are 
deep in the stalk, develop slowly, and probably serve to regenerate the 
colony when the head portion has been removed or has died down. 
Where the ectoderm is given over to storing the reserve material all the 
organs are formed from the endoderm. 
Korotneff has two important papers on the development of Salpa. 
One deals with the species S. corcUformis-zonarict and S. musculo sa-punc- 
tata, the other with S. runcinata-fusiformis. The blastomeres are of 
two kinds, formative and nutritive. According to Korotneff the 
kalymmocytes, although present among the blastomeres, do not play such 
an important part in embryogenesis as was formerly supposed. 
Heider also gives an account of the development of Salpa, his species 
being S.fusiformis. He considers it still unproved that the kalymmocytes 
have any formative as well as a nutritive role. 
Metcalf shows that in Salpa cylindrica the early disappearance of 
the eleoblast is correlated with the precocious development of the testis 
which occupies the position usually held by the eleoblast. 
Willey gives an interesting account of the process of evisceration 
which he observed in a species from New Britain, allied to Sluiter’s 
Styeloides abranehiata. If he is correct in supposing that this observa- 
tion explains the remarkable anatomical condition of Sluiter’s specimens, 
then in all probability the genus Styeloides (Sluiter) must lapse. As far 
as can be made out from Willey’s description and figures, S. eviscerans is, 
in structure, a member of the genus Styela. 
Giard & Caullery show how, in autumn, the colonies of Clavelina 
die down and disappear, with the exception of dormant winter buds, or 
masses of reserve material, of a whitish colour, in the stolons. They are 
compared to the gernmules of sponges and the statoblasts of Polyzoa. 
The reserve material is stored both in ectodermal and mesodermal cells 
of the stolon, which also contains the usual epicardial (endoderm) pro- 
longation. This modified stolon breaks up into pieces, which the follow- 
ing summer swell and grow like ordinary buds into complete individuals, 
and so the colouies are regenerated. It is a process of wintering com- 
parable to what is known in the Polyclinidce. 
Caullery has investigated the structure and the buds of some colonies 
of the genus Colella , Herdman, from Australia and Cape Horn. He finds 
the buds are of two kinds, one close to the head of the colony of ordinary 
type, and another placed further down the peduncle and characterised 
by having the outer layer of cells filled with reserve material. He thinks 
that the latter may lie dormant for long, and serve to regenerate the 
colony after the head has been amputated. 
Castle gives us what is probably the most complete and detailed 
description we yet have of the early stages in the development of au 
Ascidian. Notwithstanding the amount that has been written since 
Kowalevsky’s epoch-making work in 1866 , there still remain many 
questions unsettled in regard to the embryology of the Tunicates. 
Even on so fundamental a point as the derivation of the primary germ- 
layers in the embryo, contradictory opinions have been expressed within 
the last ten years by observers of world-wide reputation. Castle’s method 
of study, by tracing the cell-lineage, made it clear that some of the con- 
flicting statements of his predecessors arose from errors on their part, 
due to incorrect orientation of the embryos. One of the most important 
of his conclusions is that the mesoderm of Ascidians, and probably that 
