REPRODUCTIVE CELLS 13 



shell) with an underlying, thin, transparent laj'er of homogeneous proto- 

 plasm (ectoplasm), inside of which is a more opaque, granular mass 

 (endoplasm), containing a central globular vesicle (nucleus) with a dis- 

 tinct spot (nucleolus). The granules of the endoplasm are mostly of the 

 nature of yolk (deutoplasm), a sort of concentrated food-matter and store 

 of energy to be drawn on during the earlier stages of development, before 

 the organism possesses a mouth or is capable of taking up the ordinary 

 food of the oyster. Between the granules and around the nucleus is clear, 

 active cytoplasm, similar to the ectoplasm. The nucleus is bounded by 

 a membrane and contains transparent nucleoplasm, in which are embedded 

 the globular nucleolus, together with other smaller bodies called chromo- 

 somes, the latter believed to be the bearers of hereditary characters from 

 parent to offspring. There are still other structures in the egg, some of 

 which are constant and some that appear only at intervals, for the living 

 egg is an active, changing organism, although it does not make this evi- 

 dent by locomotory movements. Most of these structures are first clearly 

 exhibited after treatment with some chemical reagent or staining fluid, 

 which of course interferes with or entirely arrests the natural activities of 

 the egg. The protoplasm has a supporting network of extremely fine 

 fibres; there is a sort of cell-sap in which yolk granules may become dis- 

 solved and diffused when required to any part of the egg; there are chrom- 

 atin granules, centrosomes, and other particles. 



Sperms (spermatozoa) , when viewed under ordinary low powers of the 

 microscope (Plate V, fig. 2), such as are sufficient for the distinct recog- 

 nition of eggs, appear as mere specks or at most as minute, bright globules, 

 with a quivering movement, due to the rapid vibration of their tails, which 

 may not at first be visible. To study their size, shape, and structure re- 

 quires the highest powers and best devices of the microscope. 



The work contained herein was done with a Leitz la microscope, Abbe condenser, 

 iris diaphi-agm, triple nose-piece, oculars I, III, IV, V, objectives 2, 4, 7, i-, oil im- 

 mersion (greatest magnification 1250 diameters). Measurements by an ocular micro- 

 meter (5 mm. = 100 parts), valued by a stage micrometer (1 mm = 100 parts). 



To make the measurements more intelligible the following table is inserted show- 

 ing the actual values in micra of one of the smallest divisions of the ocular micrometer 

 (in ocular V) with each of the objectives: — 



Oc. V, Obj. 2—65 oc. mic. = 100 stage mic. = 1 mm. = 1000 fi * 

 1 15-38 // 



4-100 69 1000 69 



100 1 

 1 6-9 // 



7-100 14.5 1000^14:5 



100 1 

 1 1A5 fj. 



1/12-80 7 -100? ^1_ 



100 1 



1 875 ft 



*The Greek letter ^ is used to denote the one millionth'part of a metre, known as 

 a 'micron' (plural 'micra'). 



