430 Avery E. Lambert. 



directed to a study of the topography of the cerebral lobes in the 

 different stages of tlieir development. Hence reference will be made 

 to surface views, as figured, rather than to sections. 



By the time all of the appendages of the head and thorax have 

 arisen, at least eleven segments have been established in the ventral 

 plate. This number may be regarded as being characteristic of this 

 stage. The plate also has lengthened until it has nearly encircled 

 the yolk. (Fig. 9.) 



The nuclei of the cells composing the embryonic tissues are quite 

 small, and are closely crowded together. The nuclei which lie out- 

 side the embryonic tissue?, and which are scattered irregularly over 

 the surface of the yolk, are conspicuously larger. 



The ventral plate is divided into two lateral halves by a broad 

 median furrow (I. fr.) which begins at a point not far from the 

 anterior margin of the cephalic plate, and extends posteriorly along 

 the mid-ventral line to the margin of the caudal lobe. This furrow 

 is bridged by a thin layer of ectoderm which connects the lateral 

 halves of the embryo. The lateral halves of the ventral plate are 

 composed of clearly defined segments. 



Two segments lie anterior to those wliich bear the thoracic append- 

 ages. The posterior of these is associated with the rudimentary 

 pedipalpi ; the other bearing the rudiments of the chelicerse. In 

 this stage each of the segments of the thorax bears a pair of rudi- 

 mentary appendages. The remaining segments belong to the abdo- 

 men, and increase in number as the embryo continues to lengthen. 

 'No appendages appear, however, on the abdominal segments during 

 this stage. 



That portion of the cephalic plate which lies anterior to the che- 

 liceral segment does not present any marked differentiation at this 

 time, except that the broad, lateral areas, where the optic ganglia 

 (o. gl.) appear at a later period, are considerably thickened. In 

 transverse sections of the cephalic ])late these lateral portions are 

 seen to consist of ectoderm cells which lie several layers deep. A 

 small groove a])])ears, on eacli side of the liend, in the posterior region 

 of this thickened area (laf. gr.). These are the beginnings of the 

 invaginations which form the lateral optic vesicles. 



