the living animal 5 



Life History 



An alternation of generations in the foraminifera has been 

 known for some time. Two forms of the species are usually- 

 found together, one small in size but with a large proloculum or 

 initial chamber, the other of larger size but starting with a much 

 smaller proloculum. From the size of the proloculum, these two 

 forms were named respectively megalosphe7nc and microspheric. 

 The megalospheric form is usually much more abundant. The 

 two forms are to be looked for in all species, and it is important 

 in descriptive work to distinguish these. 



The microspheric form has a number of nuclei, often a larger 

 number than there are chambers, scattered irregularly through 

 the protoplasm of the body. There seems to be a rather definite 

 relation between the size of the nuclei and the size of the cham- 

 ber in which they occur, the larger nuclei being in the larger 

 chambers, and the reverse. Apparently these nuclei simply 

 divide in their reproduction during the growth of the test. 



When the animal attains its adult stage there is a great in- 

 crease in the number of nuclei, and the entire protoplasm either 

 leaves the test and accumulates about the exterior or is drawn 

 into the outer chambers. Finally, each nucleus gathers a mass 

 of protoplasm about itself and secretes the proloculum of a new 

 test. This newly formed proloculum is of the larger type and 

 is the first chamber of the megalospheric form, instead of being 

 of the same size as that of the microspheric parent from which 

 it was derived. The megalospheric form differs from the micro- 

 spheric form in having a single nucleus. This does not divide, 

 but moves along as new chambers are added, keeping in about 

 the middle chamber numerically. Nucleoli appear in increasing 

 numbers as the growth continues, and finally the whole nucleus 

 breaks down and a great number of minute nuclei appear. 

 These draw about themselves portions of the protoplasmic mass 

 and then divide by mitotic division. Finally, the mass leaves the 

 test in the form of flagellated zoospores. These then conjugate 

 and give rise to the small proloculum of the microspheric form, 

 thus completing the life cycle. The empty tests thus left behind 

 must form a large proportion of the dredged foraminifera, and 

 account for the great number of adult forms always present in 

 both recent and fossil collections. 



