BLASTOPORE, MESODERM AND METAMERTC SEGMENTATION. 23 
What would be the structure of a planula when this influence 
has reached back to the preinvaginated stages ? 
The ectoderm cells would grow in from either side, and en- 
croach on the solid mass of endoblast until this (i. e. endoblast) 
would be completely divided into anterior and posterior masses. 
The endoblast may be divided in this way into any number of 
separate portions (cf. below metameric segmentations). Let 
invagination now take place ; each mass of endoblast will be 
invaginated. The invaginated endoblast will grow according 
as it has been divided into endoderm or mesoderm ; such 
division may have taken place in a great many ways (vide 
mesoderm below) ; when the anterior endoblastic mass is the 
larger we have a so-called oral blastopore, e. g. Pilidium, 
Phascolosoma, and Phoronis, according to previous observers ; 
when the posterior is the larger we have an anal blastopore, 
e. g. Paludina, Serpula, and Echinodermata. Let the invagi- 
nation of the different endoblastic masses cease to be syn- 
chronous, and the primitive relations will become still more 
marked. The extreme cases are described as stomodoea and 
proctodosa. The difference between Sedgwick’s view and my 
own consists in the fact that I suppose that portions of the blas- 
topore actually exist beyond the “ parts which, in the phy- 
logenetic history in the adult, had arisen after the primitive 
mouth had completely divided into the mouth and anus.” 
II. The Masking of the Original Mode of the Origin 
of Mesodermic Formation. 
In Phoronis the original pair of diverticula are almost divided 
into two pairs. The anterior pair produce only a small proportion 
of the future mesodermic structures. Argiope is an instance of 
the anterior mesoderm being large. No posterior mesoblast has 
yet been described in this form. The mesenchyme of the praeoral 
lobe of the Hertwigs is the same anterior mesodermic diverticula 
reduced in the opposite direction. In Phoronis the anterior meso- 
derm would be described as of endoblastic origin, the middle as 
originating at the lips of the blastopore, while the posterior 
pouches would be assigned to the ectoblast. But the connec- 
