ONTOGENESEMODUS EINHEIMISCHER SORICIDEN 



1327 



Crocidura russula (White-toothed Shrew), Sorex araneus (Common Shrew) and 

 Neomys fodiens (Water Shrew). The différences in gestation periods — for Crocidura 

 30 days, for Sorex and Neomys 20 days — présents the possibility of studying the 

 phylogenetic problems in connection with thèse facts. 



A description of the development makes it possible to date material of an 

 unknown âge. The comparison of the birth situation shows that Crocidura, which 

 is characterised by a longer gestation period, is born in a more developed 

 condition. 



For the analysis of the development rate, some histogenetic processes were 

 studied and the appearance of typical characteristics were determined. The absolute 

 âge is given in ontogenetic days (OT), the âge in corrélation to birth is given in 

 postembryonic days (PT). Great attention was given to the transitory seals, which 

 are an important characteristic of development. The transitory coalescense of 

 the Hps is developed in an extraordinary manner. In Crocidura this coalescence 

 is severed on PT 6, in Sorex and Neomys on PT 1 1-12. The secondary coalescence 

 of the toes is severed in Crocidura by OT 38, in Sorex by OT 37 and in Neomys 

 by OT 39. The closure of the Meatus acusticus externus, formed by two folds of 

 the auricle, is opened in every species on OT 38-39, the close of the eyelids on 

 day 40. The synchronie opening of the transitory seals indicates a conformity 

 of the development rate in thèse species. The tooth-eruption is immediately 

 followed by the end of lactation. Thus independence is attained in Sorex on 

 OT 46, in Crocidura on OT 47 and in Neomys on OT 48. 



On the other hand, important heterochronics are found, which are certainly 

 correlated with the différent birth situation. The most important différences are 

 found in the growth process. The growth of Crocidura is retarded in the phase 

 of the prolonged embryonic development. Connected to this is the ossification, 

 studied in the skeleton of the hand. The stage of ossification equal to that of a 

 new-born Crocidura is attained by Sorex and Neomys 5 days earlier. 



The corrélation between duration of gestation and ontogenetic mode are the 

 following: The form with the larger duration of gestation and the more evolved 

 birth situation shows a remarkable retardation in growth and ossification speed. 

 Other processes of development keep the primary rate of development such as 

 the transitory seals. This corrélation explains the fact that nidifugous mammals 

 attain the moment of the opening of the eyes in relation to the ossification much 

 earlier than nidicolous mammals. 



The différences in birth situation demands a comparison with other mammals. 

 The ossification of the hand at birth in Sorex and Neomys correspond exactly 

 with that of Didelphis ( Marsupialia ) . Crocidura corresponds with the well-known 

 situation of most nidicolous Eutherians such as Mus (Housemouse). The 

 comparison of the skull-ossification shows that newborn Sorex and Neomys 

 correspond to the Opossum of PT 8, Crocidura and Mus with the Opossum of 



