December 28, 1S83.] 



SCIENCE. 



817 



mnjority of animals. Their own most impor- 

 tant addition to our knowledge apitears to me 

 to be tlieir analysis of the morpliology of mus- 

 cular tissue, l)^- which tiiey removed the most 

 important ditlieulty against the final acceptance 

 of the generalization. While we thus recognize 

 the gieat services rendered by the brothers 

 Hcrtwig, we are impelled also to express our 

 regret that they have not been more generous 

 in their acknowledgment of the achievements 

 of previous investigators ; for their theory was 

 mainly the result of a judicious combination of 

 what had been before published. To them 

 belongs the merit of ripening the fruit which 

 was already formed. 



To the mesamoeboid portion of the meso- 

 derm the Hertwigs gave the verj- appropriate 

 name of ' mesenchyma. ' For the epithelial por- 

 tion no satisfactory name has yet come into use : 

 therefore I venture to propose ' mesothelium.' 



In applying this generalization which we 

 liave been considering to vertebrates, difficul- 

 ties iiiu\ objections were encountered. To set 

 these aside, Professor Oscar Ilertwig has pub- 

 lished two special researches, the second of 

 wh'ch aiJi^eared recently, and is reproduced in 

 abstract below. 



In this review, onl^- a few salient points of 

 the history of this most important of recent 

 cmbryological discoveries are given ; but I can- 

 not close without a strong exi)ression of m}' 

 regret at being unable to notice many valuable 

 coutiibulions to the subject, — a pleasure which 

 the limited si)aee at my disposal compels me to 

 unwillingl}- forego. 



In continuation of the extended researches on the 

 origin of the mesodenii pivviously given to the world 

 by his hrotlier and himself, Oscar Hertwig now piib- 

 lishis the results of his investigalions on the de- 

 velopment of the middle layer in the frog, adding a 

 discussion of its origin in other vertebrates. The early 

 stages in the frog aredescribed with great minuteness, 

 and with far less concision and directness than we 

 should have anticipated in any of Professor Hertvrig's 

 writings. 



The essential points brought forward are the fol- 

 lowing. In the first stage, while the blastopore still 

 appears as a round white spot, the primitive darm 

 (urdarni) has the well-known form. Its inferior and 

 lateral bnundarios are the cells of the entoderm; but 

 along the dorsal line the cells offer a different histo- 

 logical character, being pigmented, an<l consisting of 

 three or four rows of small cells. In Triton, however, 

 there is a single row of high cylinder cells. This dorsal 

 band includes the anlaije of the notochord, and is 

 named by Hertwig ' chorda-entobhist.' Around the 

 blastopore the mesoderm is already present, forming 

 a paired extension running forward as a lateral mass 



on either side, and a median division lying below the 

 blastopore. Around the edges of the blastopore all the 

 layers are united : throughout the remainder of its 

 extent the mesoderm is separated by a narrow space 

 fi'om both ectoderm and entoderm. The mesoderm 

 and the chorda-entoblast are both histologically simi- 

 lar to the ectoderm; and Ilertwig, on that account, 

 believes they are both derived from the outer germ- 

 layer. (This conclusion we think is founded upon 

 an insufUcienl basis.) 



In the next staj;e the blastopore remains merely as 

 a white point, and the medullary fold* and median 

 dorsal furrow appear. The notochord is developed 

 inider the dorsal furrow as a thickening of the median 

 portion of the chorda-entoblast, which butts against 

 the ectoderm, so that the mesoderm is excluded from 

 the axial line. Ultimately the lateral portion of the 

 chorda-entoblast enters into the formation of the in- 

 testinal wall ; but in Triton tlie whole of this peculiar 

 band is changed into the chorda, which, being formed 

 by an invagination, exhibits a slit in transverse sec- 

 tions of early slages. No such slit is seen in frogs. 

 There is a fold formed at the lateral junction of the 

 chord.a-eiitoblast w'ith the rest of the entoblast; and 

 along that fold the entoderm is fused, without demar- 

 cation, with the mesoderm. Around the blastopore 

 the three layers still present essentially the same ar- 

 rangement as before; the mesoderm has grown out 

 around the whole ovum, except a small area on the 

 veutial side, where the ectoderm and entoderm (yolk) 

 are in immediate contact. 



In the next stage, when the whole length of the 

 broad medullary groove is clearly marked out, and 

 indeed in later stages also, the absolute independence 

 of the notochord of the mesoderm, and its devel- 

 opment out of and gradual separation from the 

 chorda-entoblast, are to be clearly recognized (see the 

 .accompanying figure). In the region of the bl.asto- 

 pore, where the niesoblast is continuous with the 

 other layers, there are two projecting lips, on one side 

 formed by the entoderm proper, on the other by the 

 chorda-entoblast. These lips enclose a fissure be- 

 tween them, which is a small evagination of the en- 

 teric cavity into the mesoderm. 



FronU-xl eertion ttirough a frog ovum in wliich the medullary 

 ridKt''* hnvf begun l<» appear. Knt, entoderm ; enct chorda-eD- 

 tobJHHt; cA, notochord; me, mcBodurra; ec, ectoderm; .A', ner- 

 vous oyf lem. 



In a later stage the anus is developed behind the 

 blastopore as a simple ectodermal inv.agination, the 

 bottom wall of which breaks through. No sucli re- 

 lations between the germ-layei-s have been found here, 

 or elsewhere, as around the blastopore. 



The points, then, of special importance, broughtout 

 by Hertwig, are, 1°, the existence of the median dorsal 



