EXPLANATION OF PLATES X, XI, XII, and XIII, 



List of Reference Letters. 



a. Dorso-lateral horn of white matter of brain, b. Ventro-lateral horn of 

 white matter of brain, b. app. Body cavity of appendage, a blood-space. 

 b. lat. Lateral compartment of body cavity (space formed in parietal meso- 

 derm); a blood-space, cer.ves. Cerebral vesicles or ventral appendages of 

 brain. circ. muse. Circular muscles of body wall. crur. gl. Rudiment of 

 crural gland. E. Central lobe of white matter of brain. I. s. t. 3. Tubular 

 part of nephridium of tliird somite. /. s. v. 3. Internal vesicle of nephridium 

 of third somite, neuro-musc. Network of fibres, so-called because it gives rise 

 to nerves and muscles ; it is continuous with the lateral pedal nerve, and is 

 apparently composed of a substance like the latter and the white matter of the 

 cord. It is derived from a compact mass of nuclei present in the previous 

 stages at the same point, or. pap. Oral papilla, o. s. 3. External opening of 

 nephridium of the third somite. sal.gl. Salivary gland, sl.gl. Slime-gland. 

 V. 0. Ventral organ, v. o. 1. Ventral organ of jaws. v. s. Septum separating 

 the central from the lateral compartments of the body cavity, called in earlier 

 stages the ventral sheet of somatic mesoderm. 



Figs. 1 — 4. — A series of transverse sections through the head of an embryo 

 of Peripatus Balfourii of Stage g (removed from the uterus on 12th 

 December). One side only of each section is completely drawn. The dorsal 

 ectoderm possesses a large number of highly refractile spheres, probably yolk- 

 spheres, and has contracted on to the brain so as to render indistinct the 

 mesoderm structures between. a. Dorso-lateral. b. Ventro-lateral. e. 

 Centra Icbe of white matter. 



rig. 1. Through the region of the eye and anterior lobes of the brain. 

 One side only is figured, and the nuclei of the ventral part of the brain 

 are omitted. The section goes through the optic nerve and centre of 

 the optic vesicle. Zeiss' s camera, obj. D, oc. 2. 

 Fig. 2. A little farther back, through the anterior part of the cerebral 

 vesicle {cer. ves.) and the region of junction of the two halves of the 

 brain. The dorsal mass of nuclei and the central lobe of white matter 

 have united with the corresponding structures of the opposite side. 

 Ventrally the two halves of the brain are separate. 

 Fig. 3. A little farther back, through the centre of the cerebral vesicles, 

 and still through the joined part. The dorsal mass of nuclei are absent, 

 and the white matter is broadly exposed dorsally. 

 Fig. 4. Ihrough the posterior lobes of the brain and the region of the 

 buccal cavity. The ventral organ of the jaw {v. o. 1) and the jaw are 



