XX INTRODUCTION. 



Class V. ARREST OF DEVELOPMENT. 



The most numerous and complicated groups of abnormal 

 conditions are included in this Class ; but as it is extremely 

 comprehensive and well defined, and as two or more forms 

 of arrest are sometimes united in a single malformation, it 

 is extremely doubtful whether any good would arise in the 

 substitution of several Classes in its place, at least with our 

 present knowledge of the subject. 



The various forms of arrest may be epitomized in the 

 following manner : 



I. The normal evolution of an embryonic structure 

 may fail, although its growth continues, so that an enlarge- 

 ment of the embryonic condition results. It occurs in the 

 external generative organs of the female, giving rise to a 

 similarity to the male. The condition has the appearance 

 of excessive development, but it is clearly due to the absence 

 of the usual developmental change. Doubling of the female 

 uterus by a septum, and the growth of the Miillerian duct 

 and genital cord into an enormous organ in the male, are 

 examples of this phenomenon. Some of the various kinds 

 of atresia may probably be of a similar character. 



II. The second form of arrest of development is apparently 

 due to an early arrest of growth in the lateral halves of such 

 organs as are formed by the union of two symmetrical 

 halves ; in extreme cases there is usually great accompany- 

 ing defect of development in the lateral parts. Persist- 

 ence of a cleft only, inasmuch as it is the persistence of an 

 earlier condition, is an arrest of development. Fissure of 

 the sternum, abdominal wall, palate, iris, &c. are of this 

 kind, as are also the states known as ectopia vesicae, epi- 

 spadias, and hypospadias. 



III. The third form also arises from, or is at least accom- 

 panied by, arrest of growth and atrophy, so that an organ 

 or part may be rudimentary, minute, or entirely absent. 

 When median structures fail in this way in the axis of the 

 body, the lateral parts coalesce at too early a period and 



