438 Avery E. Lambert. 



fused into a large lobe, which has become considerably thickened by 

 the continued increase in the number of its cells. The optic ganglia 

 have united to form a considerabh^ broader plate, which occupies 

 the greater part of the superficial area of the cephalic lobes. 



The sensory pits in the optic lobes have become more obscure, while 

 those of the cerebral ganglia are separated from one another, in a 

 measure, by the breaking up of the ganglia into blocks, each block 

 containing a sensory pit. This separation is brought about in such 

 a way that the cerebral ganglia seem, in surface views, to have been 

 fractured. The significance of this condition is not clear. 



The rostrum has, in this stage, lost all evidence of its double 

 origin; and is a broad, plate-like structure which entirely covers the 

 stomodcRum, The different elements of the cephalic plate have shifted 

 their positions in such a manner that the rostrum lies almost between 

 the two halves of the chelieeral segment. 



The anterior grooves (ant. gr.) have come together in the median 

 line, forming a continuous, semi-circular depression which extends 

 around the anterior margin of the cephalic plate. This groove has 

 deepened considerably, and will be refeiTed to as the anterior optic 

 invagination on account of the intimate connection w^hich exists 

 between it and the innervation of tlie median eyes. The lateral 

 grooves (lat. gr.) will be referred to as the lateral optic invagina- 

 tions, holding the same relation to the lateral eyes that the anterior 

 optic invagination holds to the median eyes. The anterior and lat- 

 eral optic invaginations form important parts of the optic ganglia. 



There is a shallow depression which appears between the lateral 

 and anterior invaginations, on each side of the head (m. o. p.). That 

 this pit may be regarded as the vestige of a more elaborate structure 

 than it now is, probably a third invagination associated with the 

 formation of the optic lobes, appears evident from the fact that it 

 occurs constantly in the cephalic plate of embryos of this stage. 



The shifting of the neuromeres of the nerve cord, and the related 

 shortening of the ventral surface of the embryo, brings a certain 

 amount of pressure to bear on the cerebral lobes which causes them 

 to be pressed over the anterior optic invagination, thus forcing the 

 lips of that infolding together. The edge of the cephalic fold (o. /.) 



