l5 



J( is ct'itamly tlif case, however, thai sonic yoimt;' eiubiyos 

 wliicli aj)[M'ar io be luisegiueiited when cxaniiixMl whole 

 with a siiupK^ h'lis are found, in sections, to he more 

 advanced than others which, are clearly segmented. It is 

 really quite impossible to say, in the present state of our 

 knowledge, what is the " normal " or " typical " proceeding 

 at this early stage. There is a stage, however, occurring 

 a few hours after fertilisation, in which the embryo con- 

 sists of a single mass of protoplasm containing several 

 protoplasmic '■ ishuids "' almost free from yolk in (\icli of 

 which there is a inicleus (PI. TIL, fig. Ki). It is really a 

 " morula " stage, althougli it may or may not exhil)it 

 mulbeny markings externally. The " islands " of proto- 

 plasm and their nuclei increase rapidly in number, and 

 soon the outlines marking the boundaries of the cells 

 become clearly dilferentiated. The nucleus of the cells 

 in these stages divides by a well-defined karyokinesis 

 (fig. 17), several beautiful achromatic spindles with their 

 chromosomes being seen in sections of every Avell-pre- 

 served embryo that is examined. A cavity makes its 

 aj^pearance in the interior of the mass of cells constituting 

 the embryo and at the same time the cells at the circiun- 

 ference become arranged in a definite row. 



In the next stage (fig. 18) a definite ectoderm is formed 

 at the periphery. This layer differs from the layer of 

 embryonal cells of the last stage in the fact that it is clear 

 and devoid of yolk globules and that the cells are ciliated. 

 Inside the ectoderm there is still a layer of un- 

 diff'erentiated embryonal tissue, laden with yolk, enclosing 

 an irregular cavity. 



Later stages than this of the larval development have 

 not yet been discovered, and it is not known how the 

 stoniodaeum and mesenteries are formed. It is probable, 

 however, that soon after the mouth is formed the larva 



