334 SUMMARY OF CURRENT RESEARCHES RELATING TO 
conditions wliicli approximate to the mode of development of compound 
forms ( Distciplia ) ; the cleavage cavity is greatly reduced, and may even 
be wanting ; in consequence of this there is no true invagination, and 
emboly is converted into pseudemboly. The result of cleavage is a 
bilaminate plakula ; the gastrula is formed by folding, the edges of the 
plakula rising up and growing towards one another ; this pseudemboly is 
due to the unequal growth of the cells of the two germinal layers. In 
the development of compound Ascidians we go further, for the archen- 
teric cavity is not formed either by emboly or pseudemboly but by 
delamination in the interior of the endodermal cell-complex. The over- 
growth of the endoderm by the ectoderm in Distaplia is not effected in 
the same way along the whole length of the embryo ; anteriorly it is 
purely epibolic, but posteriorly the dorsal endoderm-cells unite with 
the adjoining ectoderm-cells to grow round a space (pseudo-gastrula-pit) 
which is, later on, filled up by endodermal cells. This must be regarded 
as a rudimentary emboly which, notwithstanding its degeneration, 
typically repeats the conditions which obtain in social Ascidians. From 
this it is clear that the mode of development of Ascidians is parallel to 
the phylogenetic relations of their several orders. 
Dr. v. Davidoff offers the following phylum of the Chordata : — 
Mammalia 
Sauropsida 
Amphibia 
Teleostei 
— Ganoidei 
Selachii 
Primitive Fishes 
Amphioxus 
Compound Ascidians 
Social Ascidians 
Solitary Ascidians 
Primitive Chordata. 
In the study of the development of the mesoderm the author finds it 
necessary to distinguish the mesoderm of the epibolic part of the embryo 
of Distaplia (pregastric mesoderm) from that of the pseudembolic portion 
(gastric mesoderm), The elements of the latter appear early, and are 
derived from ventral parts of endoderm-cells ; they may be known as 
mother-cells or gonads of the gastric mesoderm. When pseudemboly is 
