THH i:ai;ly dkveloi'.mext (jf Tin: mai;si imai.ia. 43 

 Chapter IV. — Gimwth iif the Blastocyst and Diffeuextia- 



TIOX OF THE E:^[BKYoXAL ECTODEKM AX1> lUE ExTOHEKM. 



1 . G 1- o \v t h of t h B 1 a s t o e y s t . 



In tlie ])recLMl;ng' chapter we have seen that the cleavage 

 process in Dasynrus resnlts in the formation of a small 

 spherical vesicle^ about '-i mm. in diameter, which consists, 

 internally to the investment f.,>rnied by the a}t{)Oscil zona and 

 shell-membrane, sini])ly of a cellular wall, unilaininar throug'h- 

 out its entire extent, and enclosing a thud-tilled cavity 

 noi'mally tievoid of any cellular elements. The stage of the 

 just complete<l Idastocyst is followed iiy a period of active 

 growth ot the same, and it is a noteworthy feature in the 

 development of Dasynrus that during this time the blastocyst 

 undergoes no essential structural change, but^ renmins uni- 

 laminar until it has reacdied a diameter of from 4'o to ")"o mm. 

 Even during cleavage, the esjg of Dasynrus increases in 

 diameter, partly owing t<.) the thickening of the shell mem- 

 brane, partly, ami, indeed, inaiidy, as the result of the accumu- 

 lation of uterine fluid under pressure within the egg-envelopes, 

 but the increase due to these causes combined is relatively 

 insignificant, being only about "1 nun. As soon, however, as 

 the cellular wall of the blastocyst is completed, rapid growth 

 sets in, under the influence of the hydrostatic pressure of the 

 fluid, which tensely tills the blastocyst cavity, with the result 

 that the small relatively thick-walled Idastocyst becomes 

 converted into a large extremely thin-walled vesicle, but 

 beyond becoming veiy attenuated, the celhdar wail during 

 this period of active growth underg(_>es no essential change, 

 and retains its nnilannnar character until the blastocyst, as 

 already mentioned, has reached a duimeter of from 4'o to ■)■."> 

 mm. In vesicles of about this size there become differentiated 

 from the fornuitive cells of the upper hemisphere the em- 

 bryonal ectoderm and the ent(jderni, and this latter layer then 

 gradually spre;ids round inside the non-formative (extra- 

 embryonal ectodermal) layer of the lower hetnisphere so as to 



