114 THE PINEAL ORGAN 



understand the general position of the cephalic area of certain arthropod 

 embryos with reference to the development of the brain and that of the 

 median and lateral eyes. 



In Peripatus, when the early superficial segmentation of the ovum has 

 been completed, the central mass of yolk is covered by the blastoderm, 

 except at one part on the ventral aspect, where an invagination of cells 

 takes place. This is the blastopore, and the floor of the depression, 

 which is bounded by its thickened, involuted lips, is formed by the 

 underlying yolk. Later, the blastopore appears as an elongated groove ; 

 the sides of the groove then join in the middle part, leaving an aperture at 



cer.gr-1— 



gn.b. 



Fig. 76. — Ventral Aspect of Head of a Peripatus Embryo, showing the 

 Cerebral Grooves and Anterior Ocelli. 

 ant. : antenna. /. ; lip enclosing buccal cavity. 



cer.gr. : cerebral groove. hoc. : lateral ocellus. 



gn. : gnathite or jaw. leg 1 . : first leg. 



gn. b. : swelling at base of jaw. or. p. : oral papilla. 



" The anterior ocelli which were primarily posterior in position are carried forward 

 by the forward growth of the head." — After Sedgwick, from MacBride : 

 Textbook of Embryology, Vol. 1. 



either end. The anterior opening becomes the mouth, the posterior the 

 anus. On each side of the blastoporic groove a series of hollow meso- 

 dermic somites are formed, the cavities of which give rise to the internal 

 vesicles of the nephridia and genital ducts. At the anterior end in front 

 of the mouth is a crescentic area which gives rise to the cerebral lobes 

 and head. 



A rounded process which will give rise to the labrum is situated in 

 front of the mouth, and it is from the base of this that the two cerebral 

 grooves, previously mentioned, diverge towards the roots of the antennas. 



In Hydrophilus (Fig. 77), before the appearance of the cerebral grooves, 

 however, the whole area, including the blastoporic groove, somites, and 



