ON DEVELOP^FENT ETC. OF PHOROXIS. 529 



tivo streak," is pvodueed l)y a fusion of tlio Idastoporo lips ; tlie 

 cells along the fusion line differentiate after multi]ilieati()n into the 

 epiblast, the hypoblast, and the niesoblast. And the i'a])id gi'owth 

 of the epiblast in this region soon obliterates the groove, leaving 

 however its posteriormost portion as the " anal ])it." But such, as 

 I have tried to show, is not the case, for the so-called primitive 

 streak entirely disap])ears leaving no trace whatever, htng l)ef()re 

 tlie nephridial or anal ])it makes its appearance. Therefore there 

 exists no direct genetic relation between the primitive streak and 

 the anal pit. 



. Caldwell's view that the two nephridial pouches give off 

 the mesoblast, which eventually lines tlie posterior l)ody-cavity, 

 can not ])e sustained ; for, according to my own observations, that 

 l)ody-cavit3' with its mesoblastic lining wall is already j'jresent before 

 the nephridial pit divides into the two pouches. It is true that 

 the cells floating in the posterior l)ody-cavity arc in some sections 

 found aggregated at the 1)1 ind ends of the pouches as shown in 

 Fio". ^(S. This is a condition which mio-ht mislead one to the 

 conclusion that mesoblast cells are here in process of proliferation. 

 But solid cell accunndation in such a section is to be considered 

 as simply due to the obliteration by compressiou of the lumen of 

 the nephridial pouches. Fig. oO taken froiu an obli(|uely cut 

 sagittal section thi'ough a larva of this stage, shows no wander- 

 ing cells in front of the ]iou('lies (of which only the right one is 

 seen in the figure) ; in this case there is certainly no doul)t about 

 the matter. 



Finally as to the anus, Caldwell mentions " a solid cord of 

 cells" which he considers to be the posterior remnant of the 

 primitive streak. According to him, this acquires a lumen and 

 forms a fine canal leading fi-om the primary gut cavity to the 



