ARREST OF DEVELOPMENT. 31 



generalizations: first, that which is common to a large group of animals develops 

 in the embryo earlier than that which is special; second, from the most generalized 

 stage structures less generalized are developed, and so on until finally the most 

 special appears; third, the embryo of a given animal form, instead of passing 

 through the other given forms, separates itself from them more and more; fourth, 

 therefore, essentially the embryo of the higher forms is never like a lower form, 

 but only like its embryo. The first to point out the possible phylogenetic signific- 

 ance of these facts with perfect clearness was Fritz Miiller, in a little book entitled 

 "Fur Darwin," published in 1864. Ernst Haeckel took up this interpretation and 

 secured wider attention for it. He termed the law of recapitulation the "biogenetic 

 law."* 



The student will encounter in his practical study many illustrations of the 

 resemblances which we have been discussing, so that it is unnecessary here to do 

 more than mention a few for the purpose of illustration. In the embryos of 

 birds and mammals the pharynx forms a series of lateral pouches which we know 

 as the gill-pouches, and which develop in the same way as, resemble strikingly, 

 and are homologous with, the gill-pouches of fishes, which in the fishes give rise 

 to the so-called gill-clefts. The heart of a young mammalian embryo is a simple 

 tube with only a single continuous cavity resembling the heart of the lower fishes. 

 The embryonic kidney or Wolffian body of man resembles, and is homologous with, 

 the kidney of the frog, but it disappears almost completely before adult life. These 

 few examples may suffice. 



Arrest of Development. 



This term is used to designate not the normal, but the abnormal, cessation of 

 the ontogenetic process. It generally implies the persistence into adult life of an 

 anatomical condition, normally present in the embryo, which is typically a tem- 

 porary though essential phase of development. Usually there is no cessation of 

 the histological differentiation. It is characteristic of these anomalies that they are 

 more or less definite. 



A few illustrations may . render the matter clear. The palate is formed as 

 two shelves, which grow until they meet in the median line; sometimes they fail 

 to meet and then the adult has a "cleft palate" the tissues of which, however, 

 are as fully differentiated as those of the normal palate. In the young embryo a 

 short blood-vessel (i.e., dorsal part of the left last aortic arch) connects the pulmo- 

 nary artery with the dorsal aorta (compare page 101), but it later becomes occluded 

 and finally obliterated. Occasionally, however, it persists as an open vessel, which 

 is termed the "ductus arteriosus" in the adult. It may grow in size and its walls 

 become fully differentiated like those of an aorta. The external genitalia of the 

 male may be arrested in their development, though they continue in such cases 



* " Biogenetisches Grundgesetz." 



