218 J. A. MYERS 



but found the usual number of abdominal and inguinal pairs. 

 In the case of the rat, some authors fail to report the species 

 studied, which should always be stated in order to avoid errors 

 and confusion in making .comparisons. Lantz ('10) states that 

 the female brown rat (Mus norvegicus) has usually 12 mammae 

 • — 3 pairs of pectoral and 3 pairs of inguinal — although these 

 numbers are not constant, one or more teats frequently being 

 undeveloped. He also states that the black rat (Mus rattus) 

 and the roof rat (Mus alexandrinus) have only 10 mammae — 2 

 pairs of pectoral and 3 pairs of inguinal — with but little tendency 

 to vary. A variable number of mammary glands has also been 

 reported in many other forms, including man. Therefore, in all 

 morphological and histological work as well as experimental 

 work on the mammary gland, individual variation must be con- 

 sidered before drawing any definite conclusions. 



Cephalo-caudal sequence in development 



Henneberg's ('00) work shows that in the early stages of de- 

 velopment of the mammary gland the more cephalic or thoracic 

 glands are better developed than the caudal or abdominal and 

 inguinal glands. In fact the inguinal gland anlages remain con- 

 siderably behind the thoracic anlages. In carnivora Schultze 

 ('93) found the more cephalic mammary gland anlages earlier 

 and better developed than the posterior ones at the same age. 

 A similar condition was found in a part of the fetuses examined 

 during the present work. However, when the twenty day and 

 six hour stage is reached the difference is not so noticeable. The 

 order of sequence is therefore in accordance with the general 

 rule that those parts occupying a more cephalic position tend 

 toward earlier development than those parts occupying a more 

 caudal position. 



SUxMMARY 



1. In fetuses at fifteen days and nine hours the mammary 

 glands of the albino rat are in the club-shaped stage, the epi- 

 thelial anlage forming an ellipsoidal body attached to the epi- 

 dermis by a constricted neck. 



