EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 5) 
may under favourable conditions each of them develop into 
a perfect, full-grown individual. 
However this may be, the formation of the primitive cell- 
layers out of the cell-material derived from the segmenting 
egg-cells must now, in the first place, be considered, and, 
inverting the order generally followed, we will begin by con- 
sidering the phenomena as they present themselves in the 
A. MonopgELepHIAN AND DiIpELPHIAN MAMMALS. 
As yet only a restricted number have been investigated 
with regard to the process of cleavage and the earliest 
formation of the layers, it being no easy matter to procure 
the material. As such I mention— 
(1) Certain species of Primates,! including both monkeys 
(Macacus, Cercopithecus, a. 0.) by Selenka (99, ’00) and by 
Keibel (04), and Tarsius by myself (’02). 
(2) Lemurs (Nycticebus) by myself (’07). 
(5) Carnivores (dog and cat) by Bonnet (’97) and Duval 
(94, 795). 
(4) Chiroptera (diverse species of Vespertilii) by E. van 
Beneden and Ch. Julin (790) and by Duval (99). 
(5) Insectivora (Talpa, Erinaceus, Gymnura, Sorex, T'u- 
paja) by Heape (83), Keibel (88), and myself (’89, 790, 795, 
US) 
(6) Rodentia (Lepus, Mus, Arvicola, Cavia, Sciurus, a. 0.) 
by Hensen (’76), E. van Beneden (’80), Selenka (83, 784), 
Fraser (’82), Masius (89), Fleischmann (791), Keibel (’80), 
Duval (’92), Robinson (’92), a.o. 
(7) Ungulata (Ovis, Sus, Cervus) by Bonnet (’82), Keibel 
(93), Assheton (’98), Weysse (794), a. o. 
(8) Dermaptera (Galeopithecus) by myself. 
(9) Edentata (Manis) by myself. 
(10) Didelphia (Opossum, a.o.) by Selenka (87). 
1 Of the human subject no such early stages have as yet been brought to 
light, the earliest being those of Peters, von Heeukelom, Bryce and ‘Teacher. 
