Ö22 I. IKED A : 



scattered ai »out, is liciicefortli liroatly reduced in extent and finally, 

 as the development of the areheiiteron proo-resses, is almost o]»li- 

 tei'ated, especially aloni;' tlie dorsal and lalei'al portions of the 

 end)ryo where the ectohlast and the i;'ut come into direct contact 

 with each other (see fii^s. 29 and oO/>). 



File's. 28 a-c show three cross sections throuii-h different parts 

 of a larva of nearly tlie same stage as that represented in fig. 6, 

 in which the invagination has hecome recognizahle in the surface 

 view. Fig. 2S n passes thi'ough the central depression which be- 

 comes gi'aduallv shallower posteriorly {i'liXi^- 2<S <?* and 2.S r). As 

 these figui'cs show, the mesohlast cells are at this stage still being 

 proliferatc<l uniforndy from evei'v part of the mesentoblast and do 

 not yet form a lining epithelium to the ectohlast. When the blasto- 

 pore has taken a triangular slia])e (fig. 7) and the primaiy arclien- 

 teric cavity has somewhat bent itself towards the hind end, the 

 ]V)sterior ] »order of the blastopore has travelled a certain distance 

 in an anterior direction. Tf we examine serial sections of this 

 region, a narrow and shallow groove is detected running for a 

 short distance immediately behind, and from, the meeting point of 

 the lateral blastopore lips. Also at about this stage, a paired in- 

 vagination, the anterior diverticula of Caldwell, a]ipears along 

 the side of the anterior portion of the archenteroiL These points 

 will become clear from a consideration of figs. :10 a-r, which 

 are drawn from serial ti'ansverse sections of an eml)rvo 

 slightly older than that shown in fig. 7. Tn fig. 30 a, showing 

 the I'ight-hand side of the )>lastopore, we notice a lateral infolding 

 {mit. dir.) of the archenteric wall a short distance inside the 

 blastopore lip. Here the component cells are irregularly arranged 

 and their entire disposition reminds one of the mesentoblastic 

 layer. Indeed some indubitable mesohlast cells are found pressed 



