GERMINAL LAYERS OF TIIE EMBRYO. 
259 
(fig. 5), and to lose their cilia. These cells give rise to an 
internal parenchyma, which carries on an intra-cellular di- 
Fig. 8 . — Diagram showing the Formation of a Gastrula by Delamination. 
(From Lankester.) Fig. 1, ovum ; fig. 2, stage in segmentation ; fig. 3, 
commencement of delamination after the appearance of a central cavity ; 
fig. 4, delamination completed, mouth forming at M. In figs. 1, 2, and 
Z^Fc. is ectoplasm, and Fn. is endoplasm. In fig. 4, Fc. is epiblast, 
and Fn. hypoblast. F. food particles. 
gestion. At a later stage a central digestive cavity is supposed 
to be formed. This view of the passage from the protozoon 
to the metazoon state, though to my mind improbable in 
itself, fits in very well with the ontogeny of the lower 
Hydrozoa. 
Another view has been put forward by myself,^ to the effect 
that the amphiblastula larva of Calcispongiae may be a 
transitional form between the Protozoa and the Metazoa, 
composed of a hemisphere of nutritive amoeboid cells and a 
hemisphere of ciliated cells. The absence of such a larval 
form in the Coelenterata and higher Metazoa is opposed,, 
however, to this larva being regarded as a transitional form, 
except for the Porifera. 
It is obvious that so long as there is complete uncer- 
tainty as to the value to be attached to the early develop- 
mental processes, it is not possible to decide from these 
processes whether there is only a single Metazoon phylum 
or whether there may not be two or more such phyla. At 
* F. M. Balfour, ‘A Treatise on Comparative Embryology,’ vol. i, p. 122. 
