106 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



It is certainly the case, however, that some young embryos 

 which appear to be unsegmented when examined whole 

 with a simple lens are found, in sections, to be 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 "islands" almost free from yolk in each of 

 which there is a nucleus (PI. III., fig. 16). It is really a 

 " morula " stage, although it may or may not exhibit 

 mulberry 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 differentiated. 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 well-pre- 

 served embryo that is examined. A cavity makes its 

 appearance in the interior of the mass of cells constituting 

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

 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- 

 differentiated embiyonal 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 

 stomodseum and mesenteries are formed. It is probable, 

 however, that soon after the mouth is formed the larva 



