COMPARATIVE ANATOMY. 283 



Fig. 22. — Devklopment of Chiton Polii (after Kowalevsky). 

 A — F, Six stages in the development of the gastrula into the young Chiton, 

 sections nearly median. — G, Frontal section through stage C, oblique, from the 

 upper part of the velum to the blastopore. — H, I, K, L, Transverse sections of 

 four stages of development behind the mouth, i, Blastopore ; 2, Archenteron 

 or mid-gut ; 3, mesoderm ; 4, octoderm ; 5, velum or preoral ciliated ring ; 

 6, stomod.-eum or oesophagus ; 7, mouth ; 8, radular sac ; 9, body cavity ; 

 10, pedal gland, in one oesophagus; 11, foot; 12, anus with proctodseum ; 

 13, cerebral ganglion ; 14, pretrochal tuft ; 15, pleurovisceral cords; 16, pedal 

 cords; 17, mantle furrow; 18, eye; c, shell; c\ — cj, the seven shell-plates, 

 first formed. 



and somewhat unequal ; a coelogastrula is formed by invagination. 

 a, The blastopore of the gastrula larva marks its posterior end. 

 A pair of endoform cells near the dorsal end of the blastopore are 

 specially large. A longitudinal section shows two dorsal and two 

 ventral ectodermal cells, with larger nuclei ; these belong to a 

 double row of cells, on which is developed the preoral ciliated ring, 

 which, in Molluscs, is called the velum (Fig. 22, A). 



(b) At a later stage, the blastopore appears shifted somewhat 

 towards the ventral side, and an inward growth of ectodermal cells 

 begins at its edge ; this is the commencement of the formation of 

 the ectodermal stomodceum. At the posterior and upper edge of 

 the blastopore there is, in the figure, a cell lying between the 

 endoderm and the ectoderm ; this is, no doubt, a mesodermal 

 cell (B) 



(c) The larva elongates ; a distinct stomodasum (embryonic 

 oesophagus), leading through the blastopore into the archenteron, 

 is formed by the continuous growth inward of the ectodermal 

 cells. This organ becomes shifted still further forward along the 

 ventral surface (C). 



(dj Fig. 22, {?, is an oblique section from the anterior upper to 

 the posterior lower point through a slightly older larva, which 

 shows the stomodgeum, and, at the sides of the blastopore, the 

 ^rsl mesoderm cells. These are probably derived from the endo- 

 derm, and are symmetrically placed at the two sides of the blas- 

 topore. 



(e) A median section through the next stage (Z>) shows no 

 mesoderm cells as yet in a median place. The mouth, however, 

 appears shifted forward along the ventral side as far as the ciliated 



