MUSEUM OF COMPARATIVE ZOOLOGY. 71 



nuclei, all of which are nearer the centre than the surface of the egg. In 

 an egg still further advanced, containing at least thirty nuclei, none of 

 the cells have as yet emerged at the surface. 



These internal cells are, however, continually migrating towards the 

 periphery, and, as might be expected from other evidences of the bipolar 

 condition of the egg, make their appearance first in that region which I 

 have already designated as the animal pole. The further history of these 

 cells after they have emerged at the surface has already been described 

 under the head of surface changes. 



The problastodermic period, then, so far as I have been able to study 

 it, begins with the incomplete separation of the protoplasm into two 

 masses: one forming a thin layer at the surface — the "blastema" — 

 and the other concentrated around a nuclear structure inferred to be a 

 derivative of the germinative vesicle. The division of this nucleus is 

 accompanied by a corresponding division of the central mass of proto- 

 plasm ; a repetition of this process of division results in the formation of 

 a number of cells which, migrating to the surface, appropriate the contig- 

 uous portions of the blastema until the latter ceases to exist as a separate 

 layer ; there is no evidence that the nuclei of any of these cells arise in 

 any other way than by the repeated divisions of this single, central, first 

 segmentation-nucleus ; finally, the peripheral cells continue to subdivide 

 as well as to receive accessions from more tardily migrating elements until 

 a continuous single layer of cells — the blastoderm — envelops the egg. 



2. The second period includes the changes from the formation of the 

 blastoderm to the appearance of rudimentary appendages. 



In the eggs of Agelena juevia the blastoderm was established on the 

 third day of development, the temperature being about 23° C during the 

 day, and 19° to 20° C at night. Within certain limits* the tempera- 

 ture has a marked influence on the rapidity of the development, and one 

 can hasten or retard the growth by elevating or lowering the temperature. 

 For a day or two the blastodermic cells undergo rapid division, and are, 

 as a consequence, much reduced in size. There is a condition of the 

 blastoderm intermediate between those shown in Figs. 26 and 27, in 

 which the cells are regularly polygonal, but much larger than in Fig. 27. 



My observations on the next surface change are not entirely satisfac- 

 tory, as I have seen it in only one instance. It appeared late on the 

 third day of development, and consisted of a depression at one pole simi- 

 lar to the depression in the surface of a peach at its stem end. This is 



* The eggs are killed by a temperature higher than 30° C. 



