40 ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



by which new communities are produced. The first of these processes 

 is an asexual one. Certain cell-individuals from 20 to 35 in number and 

 situated usually in the hinder hemisphere of the colony become dis- 

 tinguished from their neighbours by their larger size. These are special 

 reproductive individuals. As a rule only eight of these proceed to carry 

 out their function. 'In this event the reproductive cell-individual con- 

 tinues to increase in size and undergoes repeated fission, dividing into 

 two, four, eight, sixteen, and so on, forming a plate of cells which as 

 the process continues curves into the form of a saucer. With increased 

 curvature the saucer deepens to form a cup and finally the mouth of the 

 cup becomes narrowed and eventually obliterated so that it takes the 

 form of a complete sphere. This is a young daughter-colony. It stands 

 out conspicuously in the substance of the mother-colony by its deeper 

 green colour, the green cell-individuals composing the daughter-colony 

 being still in close contact while those of the mother-colony are spaced 

 out by the intervening colourless jelly. The daughter-colony presently 

 makes its way from the wall of the mother-colony into its cavity which 

 is occupied by a very watery jelly, almost pure water, and here it may be 

 seen for some time gradually increasing in size, performing the ordi- 

 nary movements, and eventually bursting its way out and leading an 

 independent existence. 



This is the normal mode of increase of the Volvox communities but 

 it does not go on indefinitely. After a time a new, sexual, type of 

 reproduction takes place. This is again inaugurated by the appearance 

 of special reproductive individuals which are in this case gametocytes 

 i.e. cells which are destined to give rise to gametes. And when the 

 further development of these is watched they are seen to belong to two 

 different types male or micro-gametocytes and female or macro-gameto- 

 cytes for here for the first time we find a differentiation of two sexes. 



The macrogametocytes (Fig. 17, C, M) are at first very much like the 

 asexual reproductive individuals being like them distinguished from the 

 ordinary cell-individuals by their greater size. They are without flagella. 

 They increase greatly in size, becoming about as large as a daughter- 

 colony is before its cells become pushed apart by the secretion of jelly. 

 They are spherical in shape and are easily recognizable by their dark 

 green colour and their very dense granular cytoplasm which is laden 

 with particles of yolk or reserve food-material. The macrogametocyte 

 undergoes a process of maturation, involving divisions of its nucleus, and 

 thereafter it is capable of syngamy and we speak of it as a macrogamete 

 <>r cjrg. There are commonly about 30 macrogametes in all developed 

 in a single colony. 



