940 



SCIENCE 



[N. S. Vol. XXIX. No. 754 



complete series of graphic reconstructions 

 would seem to justify the great amount of 

 labor which it requires, and, at the reviewer's 

 suggestion, Mr. K. E. Scammon is preparing 

 such a series for the " Normentaf el " of 

 Squalus acanihias. In the Harvard Labora- 

 tory some progress has been made toward such 

 a series for the pig. This plan has been 

 partly carried out in the " Normentaf el " for 

 man, thus adding materially to its value. 

 The text figures accompany the brief descrip- 

 tions of the various embryos, which precede 

 the tables. 



In addition to the descriptions, tables, plates 

 and bibliography, there are three general dis- 

 cussions of great interest. The first (pp. 7-14) 

 is a critical account of the youngest known 

 human embryos. There has been something 

 hardly scientific in the attempt to obtain the 

 " youngest yet known " — in the description of 

 specimens " excessivement jeune," and in mono- 

 graphs on fragments and pathological debris. 

 Keibel's review shows that the two pages 

 vsrritten by von Spec, included in Peters's 

 monograph of 143 pages dealing with a single 

 specimen, is at present the most illuminating 

 account of the youngest human embryo. A 

 reconstruction of this specimen, made by 

 Keibel, is described briefly. From the study 

 of Peters's, von Spec's and Keibel's youngest 

 specimens it is inferred that the cavities of 

 the human amnion, yolk-sac and extra-embry- 

 onic coelom arise as clefts in solid masses of 

 cells; their development is illustrated in a 

 series of diagrams. An amniotic duct, such 

 as is indicated in Eternod's familiar model, is 

 not found in the younger specimens, and in 

 these there is no neurenteric canal. 



The second general chapter (pp. 80-89) in- 

 cludes a comparison of human embryos with 

 those of apes and Tarsius. It is found that 

 the youngest stages of man and the apes are 

 very similar, but that they differ materially 

 from Tarsius at a corresponding stage. Al- 

 though the human embryo is very much like 

 that of an orang, " a glance is sufficient to 

 distinguish it from any other well-knovsm 

 form." In this chapter it is stated that the 

 bend in the back of human embryos, such as 

 is seen in the reproductions of the His models 



found in most laboratories, is abnormal 



The third section (pp. 152-162) is a com- 

 parative embryological study of various 

 structures, based upon the preceding numbers 

 of the " Normentafehi." Thus it is stated 

 that the allantois in- man and the apes de- 

 velops very early, before segments have 

 formed. In Tarsius also it arises before there 

 are any segments, but later than in man and 

 the apes. It first appears in pigs of four to 

 five pairs of segments, in rabbits of about 

 eleven pairs and in chicks of more than twenty 

 pairs. Similar comparisons are made for the 

 lungs, pancreas, thyreoid gland, etc. A foun- 

 dation is thus laid for future work in com- 

 parative embryology which shall be more ac- 

 curate and detailed than anything yet realized. 

 It may be noted that in two human speci- 

 mens, a fifth pair of pharyngeal pouches was 

 identified, in one case reaching the ectoderm. 

 Fox's recent studies of the pig, cat and rabbit 

 have failed to show a fifth pouch, but Tandler 

 declares that its presence in man is not a 

 morphological speculation — it is an established 

 fact. This question is clearly one which re- 

 quires further study. In fact the great value 

 of this " Normentafel " is the stimulus and 

 aid which it affords to further research. The 

 need of early human embryos is emphasized. 

 The omission of any account of the muscular 

 and lymphatic systems is conspicuous. But 

 the great progress which has been recently 

 made in human embryology has been com- 

 pactly recorded. The work is of the utmost 

 practical value, and in a recent discrimina- 

 ting review it has been described as a " mas- 

 terpiece of scientific effort." It is the only 

 comprehensive account of strictly human 

 embryology which is now available. 



Frederic T. Lewis 



SPECIAL ARTICLES 



NOTICE OF TWO NEW HORIZONS FOR MARINE 



FOSSILS IN WESTERN PENNSYLVANIA 



Since the time of the second geological 

 survey of Pennsylvania it has been generally 

 accepted that there are three horizons at which 

 marine fossils may be found in the Conemaugh 

 series of western Pennsylvania. The oldest 

 of these is the Brush Creek limestone, about 



