36 



EMBRYOLOGY 



which are the phosphates and chlorides of potassium, sodium, and calcium. It is 

 of a semifluid, viscid consistence, and probably colloidal in nature. The living 

 cytoplasm appears to consist of a homogeneous and structureless ground-substance 

 in which are embedded granules of various types. The mitochondria are the most 

 constant type of granule and vary in form from granules to rods and threads. 

 Their function is unknown. Some of the granules are proteid in nature and prob- 

 ably essential constituents; others are fat, glycogen, or pigment granules, and are 

 regarded as adventitious material taken in from without, and hence are styled 

 cell-inclusions or paraplasm. When, however, cells have been "fixed" by reagents 

 a fibrillar or granular appearance can often be made out under a high power of the 

 microscope. The fibrils are usually arranged in a network or reticulum, to which 

 the term spongioplasm is applied, the clear substance in the meshes being termed 

 hyaloplasm. The size and shape of the meshes of the spongioplasm vary in different 

 cells and in different parts of the same cell. The relative amounts of spongioplasm 

 and hyaloplasm also vary in different cells, the latter preponderating in the young 

 cell and the former increasing at the expense of the hyaloplasm as the cell grows. 

 Such appearances in fixed cells are no indication whatsoever of the existence of 



Cell wall 



Nuclear 

 membrane 



Vacuole 



" --v,'*." ; y ^- t \ -. : -~- /' _i' A -> >-'-J 



rxS-T-V?/ tx-'.^il-x-n,J^I 



Centrosome consisting of cen- 

 Irosphere enclosing two cen- 

 trioles 



Net-knot of chromatin form- 

 ing a pseudo-nucleolus 

 Chromatin network 



Cell-inclusions (paraplasm) 



similar structures in the living, although there must have been something in the 

 living cell to give rise to the fixed structures. The peripheral layer of a cell is in 

 all cases modified, either by the formation of a definite cell membrane as in the ovum, 

 or more frequently in the case of animal cells, by a transformation, probably 

 chemical in nature, which is only recognizable by the fact that the surface of the 

 cell behaves as a semipermeable membrane. 



Nucleus. The nucleus is a minute body, imbedded in the protoplasm, and 

 usually of a spherical or oval form, its size having little relation to that of the cell. 

 It is surrounded by a well-defined wall, the nuclear membrane; this encloses the 

 nuclear substance (nuclear matrix), which is composed of a homogeneous material 

 in which is usually embedded one or two nucleoli. In fixed cells the nucleus seems 

 to consist of a clear substance or karyoplasm and a network or karyomitome. The 

 former is probably of the same nature as the hyaloplasm of the cell, but the latter, 

 which forms also the wall of the nucleus, differs from the spongioplasm of the cell 

 substance. It consists of fibers or filaments arranged in a reticular manner. These 

 filaments are composed of a homogeneous material known as linin, which stains 

 with acid dyes and contains embedded in its substance particles which have a 

 strong affinity for basic dyes. These basophil granules have been named chromatin 



