274 



better idea of the appearance of an egg at this stage than can be given 

 by words. 



It is a noticeable fact that in this stage the egg is a syncytium^ at 

 least the sixteen ectodermal cells and the mesoderm cells seem to be 

 connected by protoplasmic processes. It seems probable that so long 

 as the nuclei are below the surface that a syncytium exists, but as they, 

 with their protoplasm, begin to separate from the yolk, they become 

 distinct, as in later stage, when the cells lie upon the surface of the 

 yolk substance, no traces of connections between the various cells can 

 be made out. 



The following divisions appear to go on regularly up to the 128- 

 cell stage ; after which irregularities supervene. At about this stage a 

 concentration of both mesoderm and ectoderm cells towards the ven- 

 tral surface of the egg, which is plainly marked out in the 32-cell 

 stage by the eccentric position of the four endoderm cells, occurs ; in 

 this concentration the mesoderm cells take precedence, forming even- 

 tually a closely aggregated patch of cells, lying immediately in front of 

 the peculiar endoderm cells. A little later, in front of this the ecto- 

 derm cells aggregate to form a somewhat heart-shaped mass, the 

 head-lobes, a few scattered cells only remaining on the dorsal surface 

 of the egg. Still later the mesoderm cells begin to divide parallel to 

 the surface, so that a plug of cells is produced projecting down a short 

 distance into the yolk, forming the so called blastodisc of the Crusta- 

 cean egg. 



At about this time however a semicircle of cells may be made 

 out in some cases surrounding the front margin of the blastodisc. 

 They are more distinct in Cymotlioa than in Jaera, where however 

 they can be clearly perceived in later stages. They are ectodermal, 

 and soon begin to divide, the division being repeated always in the 

 same plane, so that each cell gives rise to a chain of cells running 

 forward from it, similar to the chains produced by the teloblasts of 

 Lumhricus or Clepsine. By this growth the teloblasts, for so these cells 

 may be termed (they correspond to the cells of the «budding Zone« 

 described for Astacus by Reichenbach), are pushed back over the 

 mesoderm cells, the semicircle becoming first straight and then con- 

 cave forwards , and the mesoderm is forced below the surface of the 

 egg. There is no invagination of the blastodisc whatever, the ectoderm 

 simply grows back over it. In the same manner the endoderm cells 

 are excluded from the surface of the egg, but in this case there is an 

 actual immigration, the cells sinking down into the interior of the 

 yolk, and becoming the » vitellophags «. The account of the segmenta- 

 tion given above necessarily implies absence of any cells in the interior 



