l24 MALCOLM LAURIE. 



cephalic lobe is seen a dark spot (fig. 32, ce. in.). These spots 

 are the cerebral invaginations. They begin in a somewhat 

 earlier stage (PI. XVI^ fig. 36) as a pair of small inpushings. 

 These extend rapidly backwards and meet in the middle line, 

 their two lumens becoming continuous. This is seen in PL 

 XVI, fig. 37 A — D, in which four transverse sections through 

 this region are figured. Owing to the strong cephalic flexure in 

 this stage the stomodseum {st.) is also shown in section. The 

 cells, both at the sides of the cephalic lobes and throughout the 

 greater part of the invaginations, are rapidly increasing in 

 number to form the cerebral ganglia. Those in the centre of 

 the cerebral lobes remain as a thin layer, and take no part in 

 the brain formation. The cells also on the dorsal side in the 

 middle, where the two invaginations have united (PI. XVI, fig. 

 37 D, oc), are more closely packed than the others, and take no 

 part in the formation of the brain. They are the beginning of 

 the retinal layer of the central eyes. 



The ventral nervous system is in much the same condition 

 histologically as it was in the last stage. The commencement 

 of its separation from the hypodermis can, however, be seen 

 (PI. XVI, fig. 35) where the hypodermis is growing over it 

 from each side as a thin layer. 



The tail segment is now divided into six somites, and extends 

 forward along the ventral surface of the body, reaching, at this 

 stage, to the third abdominal somite. The epiblast is thickened 

 on the ventral surface to form the nervous system. This is 

 not shown in fig, 35, as the section passes between two thicken- 

 ings. The cavity of the tail is occupied by a tubular extension 

 of the hypoblast (fig, 35, hy.) surrounded by mesoblast. There 

 is as yet no trace of the proctodseura. 



The coelomic spaces in the thoracic somites have not 

 developed much. Those in the abdominal somites, however 

 (PI. XVI, fig. 35, eve.), have extended enormously, and now 

 reach round almost one third of the egg. The mesoblast, ex- 

 cept in abdominal appendages, consists of two single layers of 

 cells. In the tail the coelomic spaces are not yet formed. 



Stage L. — The embryo, of which fig. 38 (PI, XVII) shows a 



