552 



None the less, in the English edition of Wiedersheim's i) Com- 

 parative Anatomy, the author felt himself obliged to say "the function 

 of this organ, though doubtless a very important one, is not under- 

 stood". And in the latest German edition of the same work 2) we 

 read: "über die Bedeutung des Organs ist nichts Sicheres bekannt". 



I have long suspected the function of the thymus to be a very 

 important one, but in spite of repeated attacks on the problem, my 

 researches had until last summer (1898) yielded little or nothing be- 

 yond a confirmation of Koelliker's brilliant discovery. 



Even now, when success has completely crowned the failures of 

 past years, at a time when the developmental history and function 

 of the thymus in Rajabatis can be demonstrated on numerous 

 preparations with the utmost certainty and ease, the problem owes 

 its solution in the first instance, not to direct attack on the organ 

 itself, but to other circumstances. These it may be well to state. 



A few years ago a very prolonged and minute investigation into 

 the developmental rise and degeneration of a transient nervous appa- 

 ratus in Raja batis culminated in the recognition of a well-defined 

 "critical period" in the development of this animal. This discovery 

 was soon extended to include other fishes, reptiles, birds, mammals, 

 and man himself. 



It led to explanations of the premature birth of most marsupials, 

 of much in the history of the placenta, and of the causes underlying 

 and determining the span of gestation and the birth-period in mammals 

 and man. 



Originally theory and hypothesis indicated the road leading to a 

 recognition of the critical period, but observation and hard work made 

 the discovery, and furnished other clues subsequently needful. In the 

 case of the thymus, also, success only rewarded the observer, when, 

 relying once more on the firm basis att'orded by the actual existence 

 of a critical period ^), he again used it as the starting point for the 



1) R. WiEDERSHEiM , Comparative Anatomy of Vertebrates. Ad- 

 apted by W. N. Paeker, 2. Edition, 1897, p. 2156. 



2) R. WiEDERSHEiM, Grundriß der vergleichenden Anatomie der 

 Wirbelthiere, 4. Aufl., 1898, p. 291. 



3) The critical period in a morphological sense has been defined 

 as „that epoch of the development, when all the parts of the organism 

 are first present as the foundations or 'Anlagen' of all the organs, it is 

 that state, when epigenesis is ended, and evolution or unfolding is 

 beginning: it is that jjoint, where the individuality of the organism is 

 first attained, when it has acquired a something setting it down as the 

 embryo of some particular form, and — the wording is important — 



