34 



CONNECTING- STALK 



between the layers is named the coelom. As the embryo takes form, it sinks 

 down, as it were, into the blastocyst, and a fold of the somatopleure comes to 

 overlap it all round. The edges of the fold ultimately meet over the embryo and 

 enclose the cavity of the amnion (figs. 48 and 49). 



The special feature about the formation of the middle layer in the Primates l is 

 that the whole extra- embryonic mesoderm is formed, at a relatively early stage, 

 before the primitive streak has appeared on the germinal disc. The precocious 

 mesoderm consists in part of cells budded off from the posterior edge of the 

 germinal area, which spread round the wall of the blastocyst and over the yolk-sac 

 (figs. 50 to 53). The tissue formed is a loosely arranged layer concerned in the 

 formation of the lining of the trophoblast and covering of the entodermic 

 sac, and from the earliest period' it forms a thick stalk of connexion (Haftstiel ; 

 embryophore) between embryonic area and trophoblast. This stalk becomes 

 vascularised at an early date, independently of the allantois, and constitutes at this 



stage a formation peculiar to the 

 Primates. In respect of its fate 

 it may be considered as corre- 

 sponding to the mesoderm which 

 extends from the posterior end 

 of the primitive streak behind 



: ,, s the germinal disc in a typical 



lower mammal.' 2 



The actual origin of this early 

 mesoderm is unknown in man. In 

 Tarsius it is derived from the ecto- 

 derm (Hubrecht). In Semnopithecus 

 nasicus Selenka considers it to be of 

 entodermic origin, but as it comes 

 from the same region of the disc 

 as in Tarsius, it is possible that an 

 earlier stage would exhibit appear- 

 ances susceptible of a similar inter- 

 pretation to that of Hubrecht. 



FIG. 50. SECTION (DIAGRAMMATIC) OF EAKLY EMBRYO OF 

 TARSIUS SPECTRUM. (After Hubrecht.) 



ect, embryonic ectoderm ; T/..S., yolk-sac; c.s., connecting 

 stalk ; p, thickened trophoblast = ectoplacenta. 



The blastocyst ia not completely imbedded in the 

 uterine mucosa, and only a portion of the trophoblast 

 therefore takes part in the formation of the placenta. The 

 ' ventral ' mesoderm covers only the posterior surface of 

 the- yolk-sac. 



While in all the Primates a 

 connecting stalk is a distinctive 

 feature of the early stages, there are some differences between the conditions in 

 Tarsius and those in apes and man, determined by the manner in which the ovum is 

 imbedded, and by the mode of formation of the amnion (cf. figs. 50, 52, and 53). In 

 Tarsius the ovum is not completely imbedded in the uterine mucous membrane, 

 and the amnio-embryonic cavity is early opened out ; only a portion of the wall of 

 the blastocyst is thickened to form the placenta, and the mesoderm passes straight 

 back from the hinder end of the embryonic plate to it, and covers, at first, the 

 posterior wall of the yolk-sac only. Owing to the maintenance of the primary inver- 

 sion in the higher Primates, on the other hand, the mesoderm is, in them, conducted 

 to the blastocyst- wall by the ' amnion stalk ' (figs. 52 and 53). It surrounds both 

 yolk-sac and amnion, so that the embryonic rudiment with its two cavities hangs 

 from the wall of the vesicle completely imbedded in the early mesoderm. The 



1 The origin of the mesoderm in the primate blastoderm has been studied in detail only in Tarsius 

 by Hubrecht. The following account is mainly founded on his description. 



The early mesoderm arising from the hinder border of the germinal area in Tarsius is named 

 by Hubrecht the ventral mesoblast, because of the theoretical relationship he believes it to bear to 

 the mesoderm of the ventral lip of the blastopore in the Amphibia. He uses the term ' mesoblast ' in 

 preference to ' mesoderm,' as he does not place the middle layer complex on the same morphological 

 plane as the ectoderm and entoderm Riickert (Hertwig's Handbuch, i. Part II.) has, on similar 

 grounds, adopted the term .ven tral mesoderm to signify the peripheral mesoderm, which appears first, and 

 springs from the hinder end of the primitive streak in the lower mammals and Amniota generally. 



