STUDIES IN MAMMALIAN EMBRYOLOGY. 351 
an early lacunar circulation in the immediate vicinity of the 
blastocyst was described. Since then he has admitted that 
the details of the formation of this lacunar tissue (as far as 
the mother’s and the embryo’s participation in the formation 
of this tissue is concerned) were not described in a way to 
satisfy its author in a later period of his investigations. 
Van Beneden’s modified views are partly expressed in a 
letter to Mathias Duval (‘Comptes Reudus de la Société de 
Biologie, tome v, p. 730; November, 1888), partly contained 
in the introduction of a paper by his pupil, J. Masius (‘ De la 
Genése du Placenta chez le Lapin,’ Archives de Biologie, 
vol. ix). The letter to Duval, dated 25th of October, 1888, was 
called forth by the publication of a short note by this author 
(‘Comptes Rendus de la Société de Biologie,’ tome v, p. 675), in 
which Duval refers to the placenta of Vespertilio murinus, 
which he had not examined personally, but which had found a 
very elaborate monographer in Professor R. Frommel, who in 
1888 published a quarto volume, ‘ Ueber die Entwickelung der 
Placenta von Myotus murinus,’ Wiesbaden, 1888. This 
volume is very brilliantly illustrated, and the plates led Duval 
to the conclusion that in the placenta of the bat in question 
processes very similar to those which he had previously pointed 
out in Rodents took place, although the author, Frommel, had 
in his text interpreted the phenomena in a different way. To 
this work of Frommel I will return by-and-by. 
Ed. van Beneden, in writing to Duval, thus corrected and 
amplified his former statements: ‘I have been occupied for 
several years with the study of the development of Vespertilio 
murinus, and I have published two notes, one concerning ‘ La 
fixation du blastocyste 4 la muqueuse utérine’ (Bull. Acad. 
Royale de Belgique, t. xv, No. i, 1888), the other, ‘La forma- 
tion et la constitution du placenta’ (ibid., No. 2). My first 
conclusions concerning the origin of the protoplasmic nucleated 
layer in which maternal blood circulates did not concur with 
your results respecting the guinea-pig and the rabbit. I 
believed I was justified in concluding that this layer arose out 
of maternal connective tissue. But you will see further on that 
