EXPLANATION OF PLATES VI, VII, VIII, & IX. 



List of Reference Letters. 



a. Anus. am. Amoeboid wandering cells in body cavity (pseudoccele). 

 a. s. ph. Septum attaching anterior part of pharynx to dorsal body wall, 

 at junction of somites 1 and 2. a. v. Anterior diverticulum of nephridium 

 of oral papilla, b. Space formed by withdrawal of endoderm from ectoderm. 

 i'. Space formed in the parietal thickening of the somites, b. app. Space 

 formed in appendage, b. be. Space formed by separation of endoderm from 

 ventral ectoderm, b. bc^. The ventral of the two spaces formed in wall of 

 somites at their ventral corner. 6. h. Space formed by separation of endoderm 

 from dorsal ectoderm, b. lot. Space formed in parietal mesoderm, b. pc. The 

 dorsal of the two spaces formed in the wall of the somites at their ventral 

 corner, br. Brain, biic. cav. Buccal cavity, c. c. Cords of cells projecting 

 into pericardial cavity, c. g. Cerebral grooves, c. g. Groove in brain, com. 

 Commissure between the two halves of the brain, c. o. n. Circumoral part 

 of central nervous system, d. n. Dividing nuclei of endoderm near the lip 

 of the blastopore, d. s. Dorsal sheet of somatic mesoderm, d. s. 1, 2, &c. 

 Dorsal part of somites 1, 2, &c. e. Eye. ec. Ectoderm, en. Endoderm. f. 

 Funnel-shaped opening of tubular part of nephridium into internal vesicle. 

 T. 1, &c. Legs. f. bl. Line of obliterated blastopore between the mouth 

 and anus. gen. Germinal nuclei, gen. d. Genital duct. gen. o. Generative 

 organ, g. and g. g. Alimentary canal. /. Jaw. j. s. Ventral organ of jaw. 

 Z. Lips. le. Internal backward projection of jaw. I. r. Limb-ridge. I. s. \, 2. &c. 

 Lateral portions of somite contained in the legs. /. s. t. 1, 2, &c. Tubular 

 portion of nephridial coelom of appendages 1, 2, &c. (segmental organ). 

 /. s. V. 1, 2, &c. Internal vesicular portion of nephridial coelom of appendages 1, 

 2, &c. M. Mouth, mb. Mesoblastic band. me. Anterior part of thickening 

 of parietal mesoderm of the somites, m. I. Muscles of internal projection 

 of jaws. m.pk. Muscular wall of pharynx, m. t. Posterior part of thickening 

 of parietal mesoderm of the somites, o. n. 1, 2, &c. The external opening 

 of nephridium of somites 1, 2, &c. or. p. Oral papilla, o. s. 3. Opening of 

 somite 3. p.g. Primitive groove, p/i. Pharynx. ^^. »^. Pharyngeal meso- 

 derm from the splanchnic walls of the anterior somites, p.p. pre-oral 

 pouch of alimentary canal, pr. Proctodaeal lining, p. st. Primitive streak. 

 R. Rectum. S. 1, 2, 3, &c. The first, second, and third somites, &c. 

 sal. gl. Salivary gland. sep. Septum dividing the lateral portion of the 

 somite from the dorsal, si. g. Slime-gland. S. o.\. Rudimentary nephridial 

 portion of somite, st. Stomodaeum or its lining. T. Tongue, v. en. Ventra 

 endoderm of alimentary canal derived from the cells intermediate between the 



