22 Dr. W. Salensky on Héckel’s Gastreea Theory. 
sesses no intestine) is overleaped. The produetion of the 
Gastrula from the Blastula-form may be regarded as an 
abridgment of development. 
Hitherto we have considered the processes of differentiation 
of the germ-lamelle, and seen that these may be referred to 
two forms—the Planula and Diblastula. From this point 
further processes occur in different manners in different 
animals, the consideration of which may assist us in obtaining 
a notion of the import of the Gastrula-stage in the ontogeny 
of animals. . 
Let us commence our examination with the animals which 
in their development pass through a Planula-stage in the pure 
form—that is to say, in the early period of their development 
present a body which consists of two or three germ-lamelle 
and has no cavity in its interior. It is in this form that the 
embryos of the vlna cacs quit their egg-envelopes ; and in 
this stage they have long been known under the above name. 
The development of the Gastrula from the Planula has been 
most accurately imvestigated by Hiickel in the Sponges, and 
described in detail in his Monograph. The phenomena of 
this process are as follows :—First of all a cavity is formed in 
the entoderm of the Planula; this stage Hickel indicates by 
the name of ‘ Planogastrula.”’ ‘The mouth then breaks into 
this cavity from without, by which the conversion of the 
Planogastrula into the Gastrula is effected. In the hydroid 
polypes this conversion has long been known. By the con- 
version of the Planula into the Gastrula the chief processes of 
the formation of the body of the Ccelenterate are already 
completed. The body in these animals (Hydroid Polypes, 
Sponges, &c.) consists during their whole life of these two 
layers which circumscribe the cavity; only the organs which 
distinguish the different groups of the Ceelenterata from each 
other (tentacles, pores, skeletal parts, &c.) are afterwards 
formed. 
It is probable that in the Turbellaria also similar simple 
processes occur in the formation of the stomachal cavity ; but 
as very little is known about their development, we cannot 
affirm this. According to Keferstem’s statements, the 
stomachal wall of these animals is produced by the differentia- 
tion of the superior layer of cells (exoderm). Unfortunately 
we do not know the developmental history of the accelous 
Turbellaria (Convoluta, Schizoprora, &e.). In their organiza= 
tion these are distinguished from the Planula only by the 
presence of the mouth. From this it may be supposed that 
the conversion of these animals from the Planula consists only 
in the perforation of the buccal orifice. 
