VIII - Studies in Tunicate development - Part III - Differential retardation and acceleration

Abstract

Accumulation of yolk within the egg has played an important part in the evolution of many groups of animals. This is especially true of the vertebrates, but comparative investigations in this series involve the study of rather widely separated types in which other evolutionary factors may have obscured the issue. In the ascidians there is such a wealth of closely related forms that it is possible to determine more or less accurately the influence of yolk accumulation, both where it is evenly distributed and where it tends to be confined to the vegetal hemisphere, and also the nature of certain factors of great significance in development and evolution unconnected with increase in egg-size. This paper contains a general account of the origin of viviparity in ascidians, the significance of cell-size, a survey of development in each family of ascidians, and a discussion of the abbreviation and telescoping of development resulting from the retarding effect of accumulation of yolk and of other factors.