at the same time irregular. Only in more advanced stages, the gradually- 

 larger blastomeres are grouped in the middle with the smaller ones around 

 and an epibolic gastrula results. Then all the borders of the cells disap- 

 pear and a syncytial mass results inside of which occurs isolation of the 

 rudiments (anlage, nobis) of the tissue's and of the organs of the larvae. 

 One must suppose also that among the remaining egg -laying monogenetic 

 trematodes the embryological development takes place in a similar fashion 

 and consequently the presence of a holoblastic egg of irregular cleavage and 

 epibolic gastrulation is characteristic for this entire group. Annong vivi- 

 parous forms the cleavage was studied by Metschnikoff (Metschnikoff , 1870), 

 Wagener (Wagener, 1860) and Kathariner (Kathariner, 1904). All these p. 92 



authors worked on the development of different species of Gyrodactylus. Among 

 the representatives of this genus the cleavage takes place just as irregularly 

 and chaotically as among Polystoma . In the cleavage of the egg of Gyro- 

 dactylus it is characteristic that in the very early stages one large blastomere 

 individualizes itself and later becomes the origin for the embryos (germ) of 

 the following generation. As is known, a very peculiar development is 

 observed among Gyrodactylus during which a number of larvae are formed 

 from a single egg, not at the same time, but gradually one after the other. 

 Gastrulation in Gyrodactylus is just as epibolic as in Polystoma. However, 

 the formation of the syncytial mass does not take place and organogenesis 

 develops by way of differentiation of cellular sections. The entire develop- 

 ment unfolds inside the uterus of the maternal organism and an adult worm 

 is born which does not differ in size from the maternal individual. A pe- 

 culiarity of the development of Gyrodactylus is that during the very early 

 stages, a second embryo forms inside the first embryo, inside of which 

 soon is incepted a third and inside the last sometimes even a fourth. A 

 number of researchers headed by Kathariner who worked specially on this 

 question consider this phenomenon as polyembryonia, and others take it 

 as one of the forms of paedogenesis. In order not to return again to Gyro- 

 dactylus, let us note that the development of the attaching apparatus among 

 the representatives of this genus coincides in basic characters with that of 

 Dactylogyridae (see development of Dactylogyrus , page 139 ). 



The periods of the development of eggs from the moment of 

 their deposition until the emergence of their free-swinnming larvae are very 

 different species and, as was shown by a number of studied, depend to a 

 large degree upon the temperature of the surrounding medium. Generally 

 one may say that in normal temperature conditions (obviously varying among 

 the different species) the development lasts from 3 to 35 days; however, 

 these periods can be considerably altered artifically; (for periods of develop- 

 ment of the separate species see the "Appendix, " pages 13 to 216). Thus, 

 according to the data of Lyman (1951a), at the temperature of 4 degrees C the 

 development of the eggs of Dactylogyrus vastator Nybelin does not take place 

 at all. At a temperature of 8 degrees the larvae of D. vastator emerge from 

 the shell of the egg on the 27th or 28th day, at 12 degrees --10th to 11th day, 

 16 degrees --6th to 7th day, 20 degrees --on the 5th day, 24 degrees --4th day. 



92 



