Nov. 22, 1883] 



NATURE 



93 



stalk, whereas a thick prominence in uhlcli no differentiation 

 can be made out, lies on the side furthest from the stall<. One 

 must, however, regard its innermost cell layer as belonging to 

 the entoderm, which also is continued into it. 



While these changes have been taking place in the embryo, 

 the "breeding-nidus" has increased considerably in size, the 

 uterine epithelium has become thicker, and consists of a finely 

 granular mass of protoplasm in which large round nuclei are 

 found, and often lie in several layers one on top of another ; 

 cell boundaries are no longer recognisable, and the pigment 

 granules, which are disappearing, still form a narrow border 

 towards the lumen of the uterus. Before and behind the breed- 

 ing-nidus is almost aUogether clo-ed by great thickenings of 

 the uterine walls. In the region of the breeding-nidus, however, 

 there is a fissure in the connective tissue wall of the uterus which is 

 probably a blood space. 



The next stage in the development of the embryo may be 

 shortly characterised as the mushroom form ; the embryo 

 becomes more clearly marked off from its stalk, and expands in 

 all directions, but most of all in the direction of the longitudinal 

 axis of the uterus, and now for the first time a bilateral symmetry 

 can with certainty be made out ; the v\hole embryo resembles a 

 mushroom with a thick stalk ; the pileus is oval, as seen from 

 above, and is a little broader at one end than at the other, and 

 on the surface, towards the narrower end, is to be seen a shallow 

 depression, which is limited towards the broader end by a slight 

 prominence. This depression corresponds to the spot where the 

 meso- and entoderms have been invaginated. There is as yet 

 no opening to connect the visceral cavity w^ith the outer world. 

 The broader end of the embryo is the head end, the stalk side 

 the back, the surface that projects into the lumen of the uterus 

 the future ventral surface. In this stage a multiple layer of 

 cells has already freed itself from the ectoderm, in front of the 

 place where the invagination took place, and it lies, passing over 

 behind into the undifferentiated cell-mas~, betv\ een the ecto- and 

 entoderm, but is marked off from both of them by a distinct 

 boundary. 



Now while the embryo increases in length, more and more 

 cells press in from the ectoderm at the spot that has been indi- 

 cated, and specialise themselves towards the front into a real 

 mesoderm, which, however, at first, and for a long time after- 

 wards, occupies only the ventral aspect and also the lateral 

 regions between ectoderm and entoderm, where it of coune 

 multiplies independently. 



In the meanwhile the thickening of the uterine epithelium has 

 gone on ; it now forms a ring, which surrounds three-quarters of 

 the circumference of .he breeding-nidus, and which as a broad 

 zone divides the breeding-nidus into two halves, where, by the 

 way, the epithelium has been thickened to a considerable though 

 a less degree ; the pigment granules have now disappeared, the 

 placenta has become very large, and the amnion, which has 

 attained a high degree of development, and which consists of 

 numerous large cells with large nuclei, lies closely applied to the 

 uterine epithelium. 



It is only when the embryo has still further increased in 

 length, the part posterior to the stalk increasing veiy quickly in 

 size, that the anus and mouth are formed, but not from a 

 common opening, the blastopore of Balfour. The anus de- 

 velops as a small fissure in the median line upon the prominence 

 in front of the spot where the invagination has taken place ; but 

 the mouth develops far further forwards as an invagination of 

 the ectoderm, consisting of only very few cells. Tliis invagina- 

 tion has an inclination obliquely from behind forwards as it pro- 

 ceeds, and reaches the iiitestine, dividing its epithelium at the 

 point of junction. This mouth invagination has as yet no lumen ; 

 this makes its appearance later, when the embryo already shows 

 its segmentation plainly. 



The first trace of this segmentation is the appearance of a 

 cavity on each side in the oldest portions of the mesoderm, i.e. 

 in the anterior extremity of the embryo, w hich splits the meso- 

 derm plates into an inner membrane adjoining the intestine, and 

 an outer one adjoining the ectoderm. These, however, are still 

 connected to each other dorsally and ventrally. Soon afterwards 

 a second pair of similar cavities develops behind, and so on from 

 before backwards. These cavities that appear in segments, and 

 which in their appearance closely resemble the original seg- 

 mental formation of a vertebrate animal, are the first rudiments 

 of the body cavity. The different structures that develop out of 

 its walls cannot be made out till later. With the exception of 

 the further growth of the posterior end of the embryo, which 



soon curves itself, rolls itself up spirally, and finally forms mani- 

 fold loops, and of the progress of the segmentatiin, and of the 

 corresponding formation of cavities in the mesoderm, no changes 

 take place in the interior of the embryo. Embryos of P. 

 Ediuardsii oi I-I'5 mm. length always present the same appear- 

 ance on cross-section : an ectoderm slightly thickened on the 

 ventral aspect, an extraordinarily thin entoderm, and between 

 them on each side a pocket of mesoderm, who^e w alls touch 

 each other in the ventral median line, and which in well-pre- 

 served embryos always are closely applied to the ectoderm, as 

 well as to the entoderm, but which always present a sharply- 

 defined boundary line. The anus is still nothing more than a 

 narrow longitudinal fissure ; the mouth has at last opened. 

 Behind the anus is situated the depression, with the place where 

 invagin.ition has taken place. 



Externally, on the other hand, a distinct segmentation of the 

 body has taken place corresponding to the cavities in the meso- 

 derm ; the anterior segment (head tegment) exceeds all the 

 others in size ; it consists of two symmetrical, spherical halves, to 

 which the other segments are connected posteriorly ; the ventral 

 aspect of the head segment contains the mouth opening. I 

 remark here that the mouth and anal opening that have been 

 mentioned must be regarded as primary in Peripatus ; the latter 

 closes at a later stage to make room for a later-developing struc- 

 ture, and the former is thrust in further by a new invagination of 

 the ectoderm, and becomes converted into the oesophageal 

 opening of the intestine. (These two observations require to be 

 checked, and I shall have to do so by examining other embryos.) 

 Each segment carries on each side a prominence which is the 

 rudiment of the limbs that are developed later. The first pair 

 of limbs is surrounded by a number of secondary papillae, and is 

 draw n into a wide mouth cavity to be utilised as a jaw ; the 

 second pair gives the papilke on whose apices the large slime 

 glands afterwards have their orifices. The tentacles are simply 

 dorsal comtinualions or prolongations of both head cavities. Now 

 at last, after the embryo has attained its full complement of seg- 

 ments, the first appearance of the nervous system can be made 

 out as a paired ventral thickening of the ectoderm, which, 

 soon separating itself from the ectoderm, extends in two 

 separate threads from one enri of the body to the other, 

 only united by the brain, which has been developed in 

 a similar manner in the head. The embryo itself, until it 

 develops a definite gullet, is intimately coimected by it- ecto- 

 derm, by means of the placenta with the maternal organism, 

 and receives its nutriment through its dorsal stalk, which can be 

 quite properly characterised as a navel-string, and which belongs 

 to the first body segment. As soon, however, a - it can swallow 

 by help of its gullet, this connection is loosened, nnd the embryo 

 now eats the food that is provided by the extr mrdinarily thick- 

 ened uterine epithelium, which is rich in protoplasmic materials. 

 At any rate, from that time forward coagulated protoplasm is 

 always to be found in the intestine of the embryo, which was 

 previously always empty. 



This is, in a few words, an abstract of the most important 

 results of my investigations up to date, which have been made 

 upon something like a hundred young embryos. I here abstain 

 for the sake of brevity from all di-cussion, but must, however, 

 call attention to the fact how little Balfour's illustrations and 

 the descriptions of the editors a^iree with the facts as they are 

 here given. I hope it will not be long before I shall he able 

 to lay before my fellow-workers my iuve ligations, «hich I hope 

 soon to complete, of these interestini; and exceedingly anomalous 

 phenomena of embryonic development, accompanied by numerous 

 illustrations. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE 



Oxford.— By a mistake last week the stiiiend of the 

 Professorship of Botany was given as 500/. instead of 700/. 

 with a residence rent free. An election will take place in 

 the course of Hilary term. The duty of the professor is to 

 lecture and give instruction in botany. He will also have 

 charge and supervision of the Botanical Garden and of the 

 botanical collections belonging to the University; and it 

 will be part of his duty to make such gardens and collec- 

 tions accessible to and available fir the ins riiciion of students 

 attending his lectures. Candidites are requested to send to the 



