DEVELOPMENT OP PHfiTGANIDS. 589 



the remainder of the section, there is a wide zone of a finely 

 granular substance in which one may observe scattered 

 nuclei. 



Summary. 



The most important results at which I have arrived may be 

 summarized as follows : 



In the earliest stages observed there were already a number 

 of germ-cells in the yolk, together with an irregular network of 

 protoplasm. All the nuclei and protoplasmic network migrate 

 to the surface and form a syncytium or " blastema/' which, by 

 the segregation of definite masses of protoplasm around each 

 nucleus, is converted into the "blastoderm," which then 

 becomes thickened at one pole to form the " ventral plate." 



From any point of the ventral plate, or serosa, cells arise by 

 budding and migrating into the yolk and form the so-called 

 " yolk-cells," the greater part of which ultimately form the epi- 

 thelial lining of the mesenteron, and hence are to be regarded as 

 true entoderm cells which have arisen by delamination from 

 the ectoderm. After the segmentation of the mesoderm certain 

 cells were observed in stages of separation from the mesodermic 

 segments, which probably migrate into the yolk, and then 

 cannot be distinguished from the ordinary yolk-cells. 



During the formation of the embryonic membranes there is 

 formed a median longitudinal invagination, resulting in the 

 production of a continuous plate of mesoderm. The groove 

 formed by this invagination quickly disappears, and then 

 follows a stage in which the ventral plate has no median de- 

 pression. Very soon, however, a second depression originates 

 along the same median line, which initiates the formation of the 

 nervous system. The latter is formed by the differentiation of 

 a pair of lateral cords arising from the division of the ectoderm 

 cells lying on either side of the neural furrow, and by the ad- 

 dition of a median infolded portion of the ectoderm, which may 

 possibly form the cross commissures. There are also transverse 

 infoldings of the ectoderm between the contiguous segments. 



