The outer follicle cells are tightly pressed against the chorion. The first polar body is extruded at fertilization. The basic pattern of mosaic development is essentially the same for all ascidians, but in Amaroucium, owing to the greater accumulation of yolk, the processes of cleavage and gastrulation are somewhat modified. The first cleavage is unequal. In the third cleavage, the dense yolky material becomes concentrated in the macromeres.
Gastrulation occurs between the sixth and seventh cleavages.
It is accomplished chiefly by epiboly, but this is accompanied by an involution of the mesoderm and a pseudo-invagination of the endoderm without the formation of an open archenteron. The closing blastopore is definitely skewed, due to a rapid growth of the right lip. C. Rate of Development Development is relatively slow, because of the large, inert yolk mass. However, no specific data pertaining to the rate of development are available, since pre-gastrula stages will not develop when removed from the parent. The lateral tail fins are well formed.